avcodec/rv10: Don't presume context to be initialized

avcodec/rv10: Don't presume context to be initialized

In case of resolution changes rv20_decode_picture_header() closes and
reopens its MpegEncContext; it checks the latter for errors, yet when
an error happens, it might happen that no new attempt at
reinitialization is performed when decoding the next frame; this leads
to crashes lateron.

This commit fixes this by making sure that initialization will always
be attempted if the context is currently not initialized.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>