[Form] Check if IntlDateFormatter constructor returned a valid object before using it
bug #11342 [2.3][Form] Check if IntlDateFormatter constructor returned a valid object before using it (romainneutron)

This PR was merged into the 2.3 branch.

Discussion
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[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

This PR replaces #11334

Commits
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ebf967d [Form] Check if IntlDateFormatter constructor returned a valid object before using it
Merge branch '2.3' into 2.4

* 2.3:
  [Translator] Use quote to surround invalid locale
  Optimize assertLocale regexp
  [Validator] Backported #11410 to 2.3: Object initializers are called only once per object
  [Translator][FrameworkBundle] Added @ to the list of allowed chars in Translator
  [Process] Reduce I/O load on Windows platform
  [Form] Check if IntlDateFormatter constructor returned a valid object before using it