Fix various uninitialized variable warnings

ffplay.c

@@ -1740,7 +1740,7 @@ static int video_thread(void *arg)
 {
     VideoState *is = arg;
     AVFrame *frame= avcodec_alloc_frame();
-    int64_t pts_int, pos;
+    int64_t pts_int = AV_NOPTS_VALUE, pos = -1;
     double pts;
     int ret;
 

libavformat/iff.c

@@ -326,6 +326,8 @@ static int iff_read_packet(AVFormatContext *s,
         buf = pkt->data;
         bytestream_put_be16(&buf, 2);
         ret = avio_read(pb, buf, iff->body_size);
+    } else {
+        av_abort();
     }
 
     if(iff->sent_bytes == 0)

libavformat/matroskadec.c

@@ -1037,7 +1037,8 @@ static void matroska_fix_ass_packet(MatroskaDemuxContext *matroska,
     char *line, *layer, *ptr = pkt->data, *end = ptr+pkt->size;
     for (; *ptr!=',' && ptr<end-1; ptr++);
     if (*ptr == ',')
-        layer = ++ptr;
+        ptr++;
+    layer = ptr;
     for (; *ptr!=',' && ptr<end-1; ptr++);
     if (*ptr == ',') {
         int64_t end_pts = pkt->pts + display_duration;