asan: _bfd_stab_section_find_nearest_line segv

The segv was on "info->strs[strsize - 1] = 0;" with strsize zero.  OK,
if strsize is zero we don't have any filenames in stabs so no useful
info.

	* syms.c (_bfd_stab_section_find_nearest_line): Exit if either
	stabsize or strsize is zero.