Should check for PCRE error on preg_replace().

See https://github.com/wp-cli/search-replace-command/issues/61#issuecomment-364783412

Should check for `null` being returned by the `preg_replace()` at [SearchReplacer.php#L105](https://github.com/wp-cli/search-replace-command/blob/v1.2.0/src/WP_CLI/SearchReplacer.php#L105) due to a PCRE error.