Fix BFD leak in dwarf2_get_dwz_file.

Fix BFD leak in dwarf2_get_dwz_file.

Previously if build_id_verify failed, dwz_bfd was cleared to NULL via
release(), but the BFD object was not destroyed.  Use reset() with
nullptr instead to delete the BFD.

gdb/ChangeLog:

	* dwarf2read.c (dwarf2_get_dwz_file): Reset dwz_bfd to nullptr
	instead of releasing ownership.