Check more completely for invalid pix_fmts in planarRgb16ToRgb16Wrapper()

This makes the conversion function more robust.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  Check more completely for invalid pix_fmts in planarRgb16ToRgb16Wrapper()

Merged-by: Michael Niedermayer <michaelni@gmx.at>