Initialize the head pointer in kld_current_sos() to NULL to avoid returning

a junk pointer and possibly causing a seg fault if we don't have any
non-kernel klds (or are unable to walk the list due to core / kernel
mismatch).

MFC after:	1 week

gnu/usr.bin/gdb/kgdb/kld.c

@@ -331,6 +331,7 @@ kld_current_sos (void)
 	char *path;
 	int error;
 
+	head = NULL;
 	prev = &head;
 
 	/*