MINOR: mux-h2: make HTX_BLK_EOM processing idempotent

We want to make sure we won't emit another empty DATA frame if we meet
HTX_BLK_EOM after and end of stream was already sent. For now it cannot
happen as far as HTX is respected, but with trailers it may become
ambiguous.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index e192cb3..4fcc339 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -4563,8 +4563,16 @@
 		count--;
 		goto end;
 	}
-
-	if (type != HTX_BLK_DATA && type != HTX_BLK_EOM)
+	else if (type == HTX_BLK_EOM) {
+		if (h2s->flags & H2_SF_ES_SENT) {
+			/* ES already sent */
+			htx_remove_blk(htx, blk);
+			total++; // EOM counts as one byte
+			count--;
+			goto end;
+		}
+	}
+	else if (type != HTX_BLK_DATA)
 		goto end;
 
 	/* Perform some optimizations to reduce the number of buffer copies.