[BUGFIX] Fix reverse order handling of MediaProvider

Fixing the variable used to test for valid providers in reverse order.

Resolves: #49136
Releases: 6.2, 6.1, 6.0
Change-Id: Ia80366150b853138fc41c09e59df993e1540b230
Reviewed-on: https://review.typo3.org/21473
Reviewed-by: Markus Klein
Tested-by: Markus Klein