flvenc: use int64_t to store offsets

Metadata currently is written only at the start of the file in normal
cases, when transcoding from a rtmp source metadata could be
written later and the offset recorded can exceed 32bit.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/flvenc.c

@@ -179,7 +179,7 @@ static int flv_write_header(AVFormatContext *s)
     AVCodecContext *audio_enc = NULL, *video_enc = NULL;
     int i;
     double framerate = 0.0;
-    int metadata_size_pos, data_size;
+    int64_t metadata_size_pos, data_size;
     AVDictionaryEntry *tag = NULL;
 
     for(i=0; i<s->nb_streams; i++){