fix the appnd() macro - it needs a block

when called below in

	if (ua_prepend[i])
		appnd(w, space, ua_prepend[i], l);

appnd would exand such that only the first line (l = strlen(r))
was conditional on ua_prepend[i] being non-null, which then would
trigger a null pointer dereference in strncpy.

This is a particularly embarassing bug.

src/dcs_varnish.c

@@ -101,6 +101,7 @@ const char * const ua_prepend[DCS_VARNISH2_NHDRS] = {
 };
 
 #define appnd(w, space, r, l)				\
+	do {						\
 		l = strlen(r);				\
 		strncpy(w, r, space);			\
 		if (l > space) {			\
@@ -109,7 +110,8 @@ const char * const ua_prepend[DCS_VARNISH2_NHDRS] = {
 			break;				\
 		}					\
 		space -= l;				\
-	w += l;
+		w += l;				\
+	} while(0)
 
 static int
 dcs_varnish_classify(dcs_ctx *ctx) {