avienc: fix overflow of audio sample count

Fixes an overflow of the sample count field within the audio stream header
chunk if audio stream data exceeds 2GB.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avienc.c

@@ -57,7 +57,7 @@ typedef struct {
 
 typedef struct  {
     int64_t frames_hdr_strm;
-    int audio_strm_length;
+    int64_t audio_strm_length;
     int packet_count;
     int entry;