libelf: Optimize elf_strptr.c validate_str by checking last char first

libelf: Optimize elf_strptr.c validate_str by checking last char first

In most cases the last char of the sectio will be zero. Check that
first before calling memrchr. This is a minor optimization in normal
cases. But it helps asan a lot by removing the memrchr call in most
cases.

https://sourceware.org/bugzilla/show_bug.cgi?id=28101

Signed-off-by: Mark Wielaard <mark@klomp.org>