mxfdec: Validate parameters to strftime

The MSVCRT version of strftime calls the invalid parameter handler
if the struct values in struct tm are invalid. In case no invalid
parameter handler is set for the process, the process is aborted.

This fixes fate failures on MSVC builds since 570af382.

Based on a patch by Hendrik Leppkes.

Signed-off-by: Martin Storsjö <martin@martin.st>