[2.3][Form] Check if IntlDateFormatter constructor returned a valid object before using it

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT

`IntlDateFormatter` constructor [may return false](http://www.php.net/manual/en/intldateformatter.create.php#refsect1-intldateformatter.create-returnvalues). This patches avoids fatal errors in these cases