[analyzer][ReturnPtrRangeChecker] Fix a false positive on end() iterator
ReturnPtrRange checker emits a report if a function returns a pointer which
points out of the buffer. However, end() iterator of containers is always such
a pointer, so this always results a false positive report. This false positive
case is now eliminated.
Patch by Tibor Brunner!
Differential Revision: https://reviews.llvm.org/D83678