avformat/mov: check color parameter type explicitly.

Fixes ticket #4387.

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

Note, Vittorio Giovara had submitted a very similar fix to me privately
a few hours before this, iam applying Jochens because it comes with a
commit message too and i had not yet applied Vittorios, but For sake
of credit, Vittorio independently solved this first
