mysqli->store_result is supposed to return false on non selects

Fixes https://github.com/facebook/hhvm/issues/2781

Produces expected result for INSERT/UPDATE and other queries that do not produce a result set.

HHVM
```
bool(false)
object(mysqli_result)#7 (3) {
  ["__result":"mysqli_result":private]=>
  resource(5) of type (mysql result)
  ["__resulttype":"mysqli_result":private]=>
  int(0)
  ["__done":"mysqli_result":private]=>
  bool(false)
}
bool(false)
```

PHP 5.5
```
bool(false)
object(mysqli_result)#7 (5) {
  ["current_field"]=>
  int(0)
  ["field_count"]=>
  int(1)
  ["lengths"]=>
  NULL
  ["num_rows"]=>
  int(1)
  ["type"]=>
  int(0)
bool(false)
}
```