MINOR: quic: Retry on qc_build_pkt() failures

This is done going to stop_build label when qc_build_pkt() fails
because of a lack of buffer room (returns -1).
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 4849a23..900b2be 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -2676,11 +2676,9 @@
 			/* If there was already a correct packet present, set the
 			 * current datagram as prepared into <cbuf>.
 			 */
-			if (prv_pkt) {
+			if (prv_pkt)
 				qc_set_dg(cbuf, dglen, first_pkt);
-				goto stop_build;
-			}
-			goto out;
+			goto stop_build;
 		default:
 			break;
 		}