Fix issue that could occur when playing mono content upmixed to stereo.

Stupid typo

[cherry-picked from 8adf652]

mythtv/libs/libmyth/audiooutputbase.cpp

@@ -990,7 +990,7 @@ int AudioOutputBase::CopyWithUpmix(char *buffer, int frames, uint &org_waud)
         {
             AudioOutputUtil::MonoToStereo(WPOS, buffer, bdFrames);
             frames -= bdFrames;
-            off = bdFrames * bpf;
+            off = bdFrames * source_bytes_per_frame;
             org_waud = 0;
         }
         if (frames > 0)