rv34: use ff_mpeg_update_thread_context only when decoder is fully initialized

MpegEncContext based decoders are only fully initialized after the first
ff_thread_get_buffer() call. The RV30/40 decoders may fail before a frame
buffer was requested. ff_mpeg_update_thread_context() fails on half
initialized MpegEncContexts. Since this can only happen before a the
first frame was decoded there is no need to call
ff_mpeg_update_thread_context().

Based on patches by John Stebbins and tested by John Stebbins.

CC: libav-stable@libav.org