Fix MSan use-of-uninitialized-value error

... introduced by 42825b68d5.  In fact,
fault-tolerant multi-scan block smoothing cannot currently be used with
the arithmetic decoder, because that decoder doesn't have any way of
distinguishing a normal end of scan from an unexpected end of scan.
Thus, this commit also modifies the change log to reset the expectations
regarding the scope of the fault-tolerant multi-scan block smoothing
feature.  If, at some point in the future, the arithmetic decoder can be
modified to detect an unexpected end of scan, then one would need only
set entropy->pub.insufficient_data = TRUE when the arithmetic decoder
encounters an unexpected end of scan in order to make fault-tolerant
block smoothing work properly with that decoder.

ChangeLog.md

@@ -57,16 +57,16 @@ and AMD CPUs, as well as a measured average overall compression speedup of
 0-23% on platforms that do not have a SIMD-accelerated Huffman encoding
 implementation.
 
-7. The block smoothing algorithm that the libjpeg API library optionally
-applies when decompressing multi-scan images is now more fault-tolerant.
-Previously, if a particular scan was incomplete, then the smoothing parameters
-for the incomplete scan would be applied to the entire output image, including
-the parts of the image that were generated by the prior (complete) scan.
-Visually, this had the effect of removing block smoothing from lower-frequency
-scans if they were followed by an incomplete higher-frequency scan.
-libjpeg-turbo now applies block smoothing parameters to each iMCU row based on
-which scan generated the pixels in that row, rather than always using the block
-smoothing parameters for the most recent scan.
+7. When decompressing progressive Huffman-encoded JPEG images, the block
+smoothing algorithm that the libjpeg API library optionally applies is now more
+fault-tolerant.  Previously, if a particular scan was incomplete, then the
+smoothing parameters for the incomplete scan would be applied to the entire
+output image, including the parts of the image that were generated by the prior
+(complete) scan.  Visually, this had the effect of removing block smoothing
+from lower-frequency scans if they were followed by an incomplete
+higher-frequency scan.  libjpeg-turbo now applies block smoothing parameters to
+each iMCU row based on which scan generated the pixels in that row, rather than
+always using the block smoothing parameters for the most recent scan.
 
 
 2.0.3

jdarith.c

@@ -731,6 +731,7 @@ bad:
   entropy->c = 0;
   entropy->a = 0;
   entropy->ct = -16;    /* force reading 2 initial bytes to fill C */
+  entropy->pub.insufficient_data = FALSE;
 
   /* Initialize restart counter */
   entropy->restarts_to_go = cinfo->restart_interval;