Fix typo causing prefix len to be wrong

When constructing the decompress side of dwaa/b compression support,
simple typo when computing the prefix length as distance between
pointers causing the length to be wrong, triggering csc decompression
values to be wrong when decompressed.

Signed-off-by: Kimball Thurston <kdt3rd@gmail.com>

src/lib/OpenEXRCore/internal_dwa_compressor.h

@@ -1600,7 +1600,7 @@ DwaCompressor_classifyChannels (DwaCompressor* me)
             prefixMap,
             me->_numChannels,
             curc->channel_name,
-            (size_t) (curc->channel_name - suffix));
+            (size_t) (suffix - curc->channel_name));
 
         for (size_t i = 0; i < me->_channelRuleCount; ++i)
         {