ICU-21662 UVector Error Handling in Regex

Clean up some oversights from commit 0ec329c. This was triggered by fuzz testing finding
a memory leak with the original commit, see https://oss-fuzz.com/testcase-detail/4656781834452992

I was unable to replicate the fuzzing failure, but reviewing the nearby code showed
some likely problems.

In this commit,
- Fix UStack::pop() to not delete the popped element when a deleter function is present.
  This was a bug, but because there were no stacks with deleters, was not causing trouble.

- Change RegexCompile::compileSet() to delete the set if it cannot be added to the internal
  vector of sets. I suspect this is the cause of the fuzzing leak - 0ec329c changed the
  behavior of UVector in the presence of errors.

- Changed RegexCompile::fSetStack to use an object deleter function. This fixes the
  leak checking at the points new elements are pushed onto this stack.

icu4c/source/common/ustack.cpp

@@ -37,10 +37,9 @@ UStack::~UStack() {}
 
 void* UStack::pop(void) {
     int32_t n = size() - 1;
-    void* result = 0;
+    void* result = nullptr;
     if (n >= 0) {
-        result = elementAt(n);
-        removeElementAt(n);
+        result = orphanElementAt(n);
     }
     return result;
 }

icu4c/source/common/uvector.h

@@ -346,6 +346,11 @@ public:
 
     inline int32_t peeki(void) const {return lastElementi();}
     
+    /**
+     * Pop and return an element from the stack.
+     * For stacks with a deleter function, the caller takes ownership
+     * of the popped element.
+     */
     void* pop(void);
     
     int32_t popi(void);

icu4c/source/i18n/regexcmp.cpp

@@ -53,7 +53,7 @@ U_NAMESPACE_BEGIN
 //
 //------------------------------------------------------------------------------
 RegexCompile::RegexCompile(RegexPattern *rxp, UErrorCode &status) :
-   fParenStack(status), fSetStack(status), fSetOpStack(status)
+   fParenStack(status), fSetStack(uprv_deleteUObject, nullptr, status), fSetOpStack(status)
 {
     // Lazy init of all shared global sets (needed for init()'s empty text)
     RegexStaticSets::initGlobals(&status);
@@ -278,11 +278,6 @@ void    RegexCompile::compile(
 
     if (U_FAILURE(*fStatus)) {
         // Bail out if the pattern had errors.
-        //   Set stack cleanup:  a successful compile would have left it empty,
-        //   but errors can leave temporary sets hanging around.
-        while (!fSetStack.empty()) {
-            delete (UnicodeSet *)fSetStack.pop();
-        }
         return;
     }
 
@@ -2431,7 +2426,11 @@ void        RegexCompile::compileSet(UnicodeSet *theSet)
             theSet->freeze();
             int32_t setNumber = fRXPat->fSets->size();
             fRXPat->fSets->addElement(theSet, *fStatus);
-            appendOp(URX_SETREF, setNumber);
+            if (U_SUCCESS(*fStatus)) {
+                appendOp(URX_SETREF, setNumber);
+            } else {
+                delete theSet;
+            }
         }
     }
 }