consolidate the encode step of transcode

Patch by Nils Goroll

src/vmod_blobcode.c

@@ -522,9 +522,9 @@ transcode(VRT_CTX, VCL_INT n, VCL_ENUM decs, VCL_ENUM encs,
 	  const char *restrict const p, va_list ap) {
 	enum encoding dec = parse_encoding(decs);
 	enum encoding enc = parse_encoding(encs);
-	struct wb_s wb;
-	ssize_t len;
 	va_list aq;
+	struct vmod_priv b;
+	VCL_STRING r;
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	if (INIT_FINI(ctx)) {
@@ -537,12 +537,11 @@ transcode(VRT_CTX, VCL_INT n, VCL_ENUM decs, VCL_ENUM encs,
 	AENC(enc);
 
 	if (dec == IDENTITY) {
-		struct vmod_priv b;
 		va_copy(aq, ap);
 		const struct vmod_priv *bb = decode_id_inplace(&b, n, p, aq);
 		va_end(aq);
 		if (bb != NULL) {
-			VCL_STRING r = encode(ctx, enc, bb);
+			r = encode(ctx, enc, bb);
 			return (r);
 		}
 	}
@@ -558,13 +557,15 @@ transcode(VRT_CTX, VCL_INT n, VCL_ENUM decs, VCL_ENUM encs,
 		return "";
 	/* XXX: handle stack overflow? */
 	char buf[l];
+	b.free = NULL;
+	b.priv = buf;
 
 	errno = 0;
 	va_copy(aq, ap);
-	len = func[dec].decode(dec, buf, l, n, p, aq);
+	b.len = func[dec].decode(dec, buf, l, n, p, aq);
 	va_end(aq);
 
-	if (len == -1) {
+	if (b.len == -1) {
 		err_decode(ctx, p);
 		return NULL;
 	}
@@ -574,28 +575,12 @@ transcode(VRT_CTX, VCL_INT n, VCL_ENUM decs, VCL_ENUM encs,
 	 * legal, just return the concatenated string.
 	 */
 	if (n == -1 && enc == dec) {
-		VCL_STRING s;
-		s = VRT_String(ctx->ws, NULL, p, ap);
-		return s;
-	}
-
-	if (wb_create(ctx->ws, &wb) == NULL) {
-		ERRNOMEM(ctx, "cannot encode after decode");
-		return NULL;
+		r = VRT_String(ctx->ws, NULL, p, ap);
+		return r;
 	}
 
-	len = func[enc].encode(enc, wb_buf(&wb), wb_space(&wb), buf, len);
-	if (len == -1) {
-		ERRNOMEM(ctx, "cannot encode after decode");
-		wb_reset(&wb);
-		return NULL;
-	}
-	if (len == 0) {
-		wb_reset(&wb);
-		return "";
-	}
-	wb_advance(&wb, len);
-	return wb_finish(&wb, NULL);
+	r = encode(ctx, enc, &b);
+	return (r);
 }
 
 VCL_STRING __match_proto__(td_blobcode_transcode)