test: stacksmash: Make sure to change the value of the written byte

If the low byte of the stack cookie happens to be zero, we
previously wouldn't trigger any error.

This avoids accidentally failing to trigger the stack protector
once per 256 runs, roughly.

test/stacksmash.c

@@ -20,7 +20,7 @@
 
 __attribute__((noinline))
 void func(char *ptr, int idx) {
-    ptr[idx] = 0;
+    ptr[idx] ^= 1;
 }
 
 int main(int argc, char *argv[]) {