Handler target_get_gdb_reg_list() better.

That function might change from NULL reg_list but then return failure.
In that case reg_list shouldn't be freed.

Change-Id: I5380630c871d056fb52e25bda16836e346bd74b2

src/server/gdb_server.c

@@ -2399,6 +2399,7 @@ static int gdb_target_description_supported(struct target *target, int *supporte
 			&reg_list_size, REG_CLASS_ALL);
 	if (retval != ERROR_OK) {
 		LOG_ERROR("get register list failed");
+		reg_list = NULL;
 		goto error;
 	}