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 43d8cf4d45 did for regular
images.

ChangeLog.txt

@@ -22,6 +22,9 @@ decoder only if there are > 512 bytes of data in the input buffer.
 [3] Fixed a memory leak in tjunittest encountered when running the program
 with the -yuv option.
 
+[4] Fixed an issue whereby a malformed motion-JPEG frame could cause the "fast
+path" of libjpeg-turbo's Huffman decoder to read from uninitialized memory.
+
 
 1.4.2
 =====

jstdhuff.c

@@ -41,6 +41,7 @@ add_huff_table (j_common_ptr cinfo,
     ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
 
   MEMCOPY((*htblptr)->huffval, val, nsymbols * sizeof(UINT8));
+  MEMZERO(&((*htblptr)->huffval[nsymbols]), (256 - nsymbols) * sizeof(UINT8));
 
   /* Initialize sent_table FALSE so table will be written to JPEG file. */
   (*htblptr)->sent_table = FALSE;