ICU-20776 Checks for nullness in UMutex::lock()

Adds `U_ASSERT` check before using `m->lock()` to make allocation issues
more apparent at least in debug builds.

There is probably quite a few places like this, but let's try fixing
broken windows.

icu4c/source/common/umutex.cpp

@@ -98,6 +98,7 @@ std::mutex *UMutex::getMutex() {
             gListHead = this;
         }
     }
+    U_ASSERT(retPtr != nullptr);
     return retPtr;
 }
 

icu4c/source/common/umutex.h

@@ -240,7 +240,8 @@ private:
     static UMutex *gListHead;
 
     /** Out-of-line function to lazily initialize a UMutex on first use.
-     * Initial fast check is inline, in lock().
+     * Initial fast check is inline, in lock().  The returned value may never
+     * be nullptr.
      */
     std::mutex *getMutex();
 };