libswresample/audioconvert: Fix undefined NULL + 0

Affected 26 FATE tests like swr-resample_async-s16p-44100-8000.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libswresample/audioconvert.c

@@ -237,10 +237,10 @@ int swri_audio_convert(AudioConvert *ctx, AudioData *out, AudioData *in, int len
         const int ich= ctx->ch_map ? ctx->ch_map[ch] : ch;
         const int is= ich < 0 ? 0 : (in->planar ? 1 : in->ch_count) * in->bps;
         const uint8_t *pi= ich < 0 ? ctx->silence : in->ch[ich];
-        uint8_t       *po= out->ch[ch];
-        uint8_t *end= po + os*len;
+        uint8_t *end, *po = out->ch[ch];
         if(!po)
             continue;
+        end = po + os * len;
         ctx->conv_f(po+off*os, pi+off*is, is, os, end);
     }
     return 0;