don't try to fetch status of minus one message

this would happen if we were trying to find newly pushed messages, but
none actually arrived.
as imap's ranges are not ordered, this would actually fetch one message.

src/drv_imap.c

@@ -1612,6 +1612,8 @@ imap_load( store_t *gctx, int minuid, int maxuid, int newuid, int *excs, int nex
 				sprintf( buf, "%d:%d", minuid, newuid - 1 );
 				if (imap_submit_load( ctx, buf, 0, sts ) < 0)
 					goto done;
+				if (newuid > maxuid)
+					goto done;
 				sprintf( buf, "%d:%d", newuid, maxuid );
 			} else {
 				sprintf( buf, "%d:%d", minuid, maxuid );