micro-optimize backref when the reference is the subject tail

test is already included in c01.vtc

src/vmod_re.c

@@ -237,8 +237,11 @@ backref(VRT_CTX, VCL_INT refnum, VCL_STRING fallback, struct vmod_priv *task)
 	start = ov->subject + ov->ovector[refnum];
 	len = ov->ovector[refnum+1] - ov->ovector[refnum];
 	assert(len <= ov->ovector[1] - ov->ovector[0]);
-	WS_Assert_Allocated(ctx->ws, start, len);
-	substr = WS_Printf(ctx->ws, "%.*s", len, start);
+	WS_Assert_Allocated(ctx->ws, start, len + 1);
+	if (start[len] == '\0')
+		substr = start;
+	else
+		substr = WS_Printf(ctx->ws, "%.*s", len, start);
 	if (substr == NULL) {
 		VSLb(ctx->vsl, SLT_VCL_Error,
 		     "vmod re: insufficient workspace");