Add a got_picture flag to MJpegDecodeContext which indicates if its picture
element is valid. Skip the code handling SOS and EOI if not, since it can not
work without a valid AVPicture.
This fixes a crash with mjpeg/smclockmjpeg.avi.1.0 from issue 1240 where the
decoder returned an invalid AVPicture.

Originally committed as revision 19342 to svn://svn.ffmpeg.org/ffmpeg/trunk
