MINOR: h3: fix compiler warning variable set but not used

Some variables were only checked via BUG_ON macro. If compiling without
DEBUG_STRICT, this instruction is a noop. Fix this by using an explicit
condition + ABORT_NOW.

This should fix the github issue #1549.
diff --git a/src/h3.c b/src/h3.c
index 5f2757f..965812e 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -211,11 +211,14 @@
 	htx = htx_from_buf(appbuf);
 
 	htx_space = htx_free_data_space(htx);
-	BUG_ON(!htx_space || htx_space < len);
+	if (!htx_space || htx_space < len) {
+		ABORT_NOW(); /* TODO handle this case properly */
+	}
 
 	htx_sent = htx_add_data(htx, ist2(b_head(buf), len));
-	/* TODO handle full appbuf */
-	BUG_ON(htx_sent < len);
+	if (htx_sent < len) {
+		ABORT_NOW(); /* TODO handle this case properly */
+	}
 
 	if (fin)
 		htx->flags |= HTX_FL_EOM;
@@ -264,12 +267,12 @@
 		case H3_FT_DATA:
 			ret = h3_data_to_htx(qcs, rxbuf, flen, last_stream_frame);
 			/* TODO handle error reporting. Stream closure required. */
-			BUG_ON(ret);
+			if (ret) { ABORT_NOW(); }
 			break;
 		case H3_FT_HEADERS:
 			ret = h3_headers_to_htx(qcs, rxbuf, flen, last_stream_frame);
 			/* TODO handle error reporting. Stream closure required. */
-			BUG_ON(ret);
+			if (ret) { ABORT_NOW(); }
 			break;
 		case H3_FT_PUSH_PROMISE:
 			/* Not supported */