Fix 'obj' may be used uninitialized warning in symtab.c:matching_obj_sections.

Fix warning:

gdb/symtab.c: In function ‘int matching_obj_sections(obj_section*, obj_section*)’:
gdb/symtab.c:1024:12: warning: ‘obj’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   if (obj->separate_debug_objfile_backlink != NULL

2019-01-12  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* symtab.c (matching_obj_sections): Initialize obj,
	declare it closer to its usage.