audio: Avoid accumulation errors in resampler.

Fixes #6391.

src/audio/SDL_audiocvt.c

@@ -247,7 +247,7 @@ SDL_ResampleAudio(const int chans, const int inrate, const int outrate,
             *(dst++) = outsample;
         }
 
-        outtime += outtimeincr;
+        outtime = outtimeincr * i;
     }
 
     return outframes * chans * sizeof (float);