2 execontexts in an hash table chain are not necessarily the same size.
So, ensure that when size differs, we do not start to compare them,
as this could otherwise cause a read buffer overrun



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14097 a5019735-40e9-0310-863c-91ae7b9d1cf9