Avoid copying for enc == dec if we can

Patch from Nils Goroll

src/vmod_blobcode.c

@@ -572,9 +572,21 @@ transcode(VRT_CTX, VCL_INT n, VCL_ENUM decs, VCL_ENUM encs,
 
 	/*
 	 * If the encoding and decoding are the same, and the decoding was
-	 * legal, just return the concatenated string.
+	 * legal, just return the string, if there was only one in the
+	 * STRING_LIST, or else the concatenated string.
 	 */
 	if (n == -1 && enc == dec) {
+		const char *q, *pp = p;
+		va_copy(aq, ap);
+		q = find_nonempty_va(&pp, ap);
+		va_end(aq);
+
+		if (pp == vrt_magic_string_end)
+			return "";
+
+		if (q == vrt_magic_string_end)
+			return pp;
+
 		r = VRT_String(ctx->ws, NULL, p, ap);
 		return r;
 	}
@@ -598,7 +610,7 @@ vmod_transcode(VRT_CTX, VCL_ENUM decs, VCL_ENUM encs,
 
 VCL_STRING __match_proto__(td_blobcode_transcode_n)
 vmod_transcode_n(VRT_CTX, VCL_INT n, VCL_ENUM decs, VCL_ENUM encs,
-	       const char *p, ...) {
+		 const char *p, ...) {
 	va_list ap;
 	VCL_STRING r;