checks: Fix crash with multiple source annotations

Commit 3616b9a811 ("checks: Use source position information for check
failures") causes crashes when there's a check message with multiple
source annotations. Drop the errant addition to the str pointer left
over from the previous version.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

checks.c

@@ -123,7 +123,7 @@ static inline void  PRINTF(5, 6) check_msg(struct check *c, struct dt_info *dti,
 			pos = pos->next;
 
 			file_str = srcpos_string(pos);
-			str += xasprintf_append(&str, "  also defined at %s\n", file_str);
+			xasprintf_append(&str, "  also defined at %s\n", file_str);
 			free(file_str);
 		}
 	}