Validate video descriptor before checking still picture bit.

Check to ensure that the first descriptor is a video_stream descriptor
(value 0x02) before parsing its contents. I have four test samples,
only one of which contains a video_stream descriptor in the PMT. The
others contain a system clock descriptor (0x11), a maximum bitrate
descriptor(0x14), and a private descriptor(0x86).  Test cases for all
four samples will be forthcoming.

This code still only handles the first descriptor of possibly multiple
stream descriptors. This code still needs to be updated to scan
through all the possible descriptors.

mythtv/libs/libmythtv/mpeg/mpegtables.cpp

@@ -624,6 +624,7 @@ bool ProgramMapTable::IsStillPicture(const QString& sistandard) const
         if (IsVideo(i, sistandard))
         {
             return StreamInfoLength(i) > 2 &&
+                   (StreamInfo(i)[0] == DescriptorID::video_stream) &&
                    ((StreamInfo(i)[2] & kStillPictureFlag) != 0);
         }
     }