when joining the UA string from additional headers, seperate with a space

dont truncate the last character

src/dcs_varnish.c

@@ -158,20 +158,32 @@ dcs_varnish_classify(dcs_ctx *ctx) {
 	} while(0);
 	ua = uabuf;
 
+	if (space == 0)
+		goto nospc;
+
 	for (i = 0; i < DCS_VARNISH2_NHDRS; i++) {
 		r = DCS_GetHdr(ctx, &hdrs[i]);
 		if (r && *r) {
-			if (ua_prepend[i])
+			if (ua_prepend[i]) {
 				appnd(w, space, ua_prepend[i], l);
+			} else {
+				// space==0 will heave left the loop
+				assert(space > 0);
+				*w++ = ' ';
+				w[1] = '\0';
+				if (--space == 0)
+					break;
+			}
 			appnd(w, space, r, l);
 		}
 	}
 
+  nospc:
 	/* ensure the string is always terminated */
 	if (w == (uabuf + UA_LIMIT))
 		uabuf[UA_LIMIT] = '\0';
 	else
-		w[-1] = '\0';
+		*w = '\0';
 
 	assert(ua == uabuf);
 	assert(w > uabuf);