old_val pointer in README example not initialized.

[vassvik]

The old_val pointer in the README example isn't initialized. This results in the last conditional falsely firing on some compilers.

charchar *key = strdup( *key = st "my key");
char *val = strdup("my val");

// if a caller knows the value doesn't already exist
// BEWARE: using it this way can cause memory leaks!
if (!rsht_put(ht, key, val, NULL)) {
  // handle out of memory error
}

// if a caller wants to know the previous value and free it
void *old_val; // Should be initialized to zero
if (!rsht_put(ht, key, val, &old_val)) {
  // handle out of memory error
  free(key);
}
if (old_val) {
  free(key); // if the value existed, we can free the key we just used
  free(old_val);
}