MINOR: mux-h2: remove useless check for empty frame length in h2s_decode_headers()

This test for an empty frame was already performed in the callers, there
is no need for checking it again.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 1608279..dae53c6 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -3143,12 +3143,6 @@
 	int wrap;
 	int try = 0;
 
-	if (!h2c->dfl) {
-		/* RFC7540#4.2 */
-		h2c_error(h2c, H2_ERR_FRAME_SIZE_ERROR); // empty headers frame!
-		goto fail;
-	}
-
 	if (b_data(&h2c->dbuf) < h2c->dfl && !b_full(&h2c->dbuf))
 		return 0; // incomplete input frame