fix a possible free of a uninitialize variable in paste()

dmenu.c

@@ -467,10 +467,12 @@ paste(void)
 	Atom da;
 
 	/* we have been given the current selection, now insert it into input */
-	XGetWindowProperty(dpy, win, utf8, 0, (sizeof text / 4) + 1, False,
-	                   utf8, &da, &di, &dl, &dl, (unsigned char **)&p);
-	insert(p, (q = strchr(p, '\n')) ? q - p : (ssize_t)strlen(p));
-	XFree(p);
+	if (XGetWindowProperty(dpy, win, utf8, 0, (sizeof text / 4) + 1, False,
+	                   utf8, &da, &di, &dl, &dl, (unsigned char **)&p)
+	    == Success && p) {
+		insert(p, (q = strchr(p, '\n')) ? q - p : (ssize_t)strlen(p));
+		XFree(p);
+	}
 	drawmenu();
 }