BUG/MINOR: h3: ignore unknown frame types

When receiving an unknown h3 frame type, the frame must be discarded
silently and the processing of the remaing frames must continue. This is
according to the HTTP/3 draft34.

This issue was detected when using the quiche client which uses GREASE
frame to test interoperability.

src/h3.c

@@ -212,9 +212,10 @@ static int h3_decode_qcs(struct qcs *qcs, void *ctx)
 			/* Not supported */
 			break;
 		default:
-			/* Error */
-			h3->err = H3_FRAME_UNEXPECTED;
-			return -1;
+			/* draft-ietf-quic-http34 9. Extensions to HTTP/3
+			 * unknown frame types MUST be ignored
+			 */
+			h3_debug_printf(stderr, "ignore unknown frame type 0x%lx\n", ftype);
 		}
 		b_del(rxbuf, flen);
 	}