examples/x11.c: fix result of operation is garbage or undefined

Fix use of uninitialized structure w_size_bck.
Detected by clang scan in	line 386, column 28.

example/x11.c

@@ -376,6 +376,9 @@ main (int argc, char *argv[])
         return -1;
     }
 
+    memset(&w_size, 0, sizeof(struct winsize));
+    memset(&w_size_bck, 0, sizeof(struct winsize));
+
     while (1) {
 
         FD_ZERO(&set);