[HttpFoundation] UploadedFile - moved a security check from move() to isValid()

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [yes slightly]
| Deprecations? | [no]
| Tests pass?   | [yes]
| License       | MIT

Fixed and reopened against 2.0 as per @vicb comments in #6779.