Ensure that default Huffman tables are initialized

This prevents a malformed motion-JPEG frame (MJPEG frames lack Huffman
tables) from causing the "fast path" of the Huffman decoder to read
uninitialized memory.  Essentially, this is doing the same thing for
MJPEG frames as 43d8cf4d4572fa50a37cccadbe71b9bee37de55d did for regular
images.
2 files changed