Fix NULL pointer dereference in S3CalculateRandomizedFields (#284)

If the sound has been muted (!gS3_enabled) during game data loading, it
is then possible to unmute it during gameplay and request a sound effect
which doesn't have a sample loaded (most prominent for pratcam sfx).
While logic in `S3StartSound` accommodates for such a case by loading
the missing sample, it first calls `S3CalculateRandomizedFields`, which
triggers a NULL pointer dereference on platforms with memory protection.
This bug is most likely an overlook from the DOS era.

Fix this by checking for NULL pointer before use.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>

src/S3/CMakeLists.txt

@@ -18,6 +18,9 @@ else()
         /wd4996
     )
 endif()
+if(DETHRACE_FIX_BUGS)
+    target_compile_definitions(s3 PRIVATE DETHRACE_FIX_BUGS)
+endif()
 
 if(IS_BIGENDIAN)
     target_compile_definitions(s3 PRIVATE BR_ENDIAN_BIG=1)

src/S3/audio.c

@@ -962,6 +962,13 @@ void S3CalculateRandomizedFields(tS3_channel* chan, tS3_descriptor* desc) {
     chan->left_volume = vol;
     chan->right_volume = vol;
     if (desc->type == eS3_ST_sample) {
+#if defined(DETHRACE_FIX_BUGS)
+    /* Avoid a possible NULL pointer dereference. */
+    if (desc->sound_data == NULL) {
+        chan->rate = desc->min_pitch;
+        return;
+    }
+#endif
         chan->rate = S3IRandomBetweenLog(desc->min_pitch, desc->max_pitch, ((tS3_sample*)desc->sound_data)->rate);
     }
 }