mjpegdec: extend check for incompatible values of s->rgb and s->ls

This can happen if s->ls changes from 0 to 1, but picture allocation is
skipped due to s->interlaced.

In that case ff_jpegls_decode_picture could be called even though the
s->picture_ptr frame has the wrong pixel format and thus a wrong
linesize, which results in a too small zero buffer being allocated.

This fixes an out-of-bounds read in ls_decode_line.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
