segv in bfd_elf_get_str_section

Attempting to write a termination NUL to PROT_READ mmap'd memory was
a silly idea.

	PR 32109
	* elf.c (bfd_elf_get_str_section): Don't write terminating NUL
	if missing.
	* libbfd.c (_bfd_munmap_readonly_temporary): Correct comment.