MINOR: h3: fix possible invalid dereference on htx parsing

The htx variable is only initialized if we have received a HTTP/3
HEADERS frame. Else it must not be dereferenced.

This should fix the compilation on CI with gcc.
  src/h3.c: In function ‘h3_decode_qcs’:
  src/h3.c:224:14: error: ‘htx’ may be used uninitialized in this function
  [-Werror=maybe-uninitialized]
    224 |   htx->flags |= HTX_FL_EOM
diff --git a/src/h3.c b/src/h3.c
index 970b957..79d7460 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -100,7 +100,7 @@
 {
 	struct buffer *rxbuf = &qcs->rx.buf;
 	struct h3 *h3 = ctx;
-	struct htx *htx;
+	struct htx *htx = NULL;
 	struct htx_sl *sl;
 	struct conn_stream *cs;
 	struct http_hdr list[global.tune.max_http_hdr];
@@ -220,8 +220,10 @@
 		b_del(rxbuf, flen);
 	}
 
-	if (fin && !b_data(rxbuf))
-		htx->flags |= HTX_FL_EOM;
+	if (htx) {
+		if (fin && !b_data(rxbuf))
+			htx->flags |= HTX_FL_EOM;
+	}
 
 	return 0;