avformat/kvag: Check sample_rate

Fixes: Division by 0
Fixes: -copyts -start_at_zero -itsoffset 00:00:01 -itsscale 1 -ss 00:00:02  -i zgclab/ffmpeg_crash/poc1 output.mp4

Found-by: Wang Dawei and Zhou Geng, from Zhongguancun Laboratory
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/kvag.c

@@ -38,7 +38,7 @@
 typedef struct KVAGHeader {
     uint32_t    magic;
     uint32_t    data_size;
-    uint32_t    sample_rate;
+    int    sample_rate;
     uint16_t    stereo;
 } KVAGHeader;
 
@@ -70,6 +70,9 @@ static int kvag_read_header(AVFormatContext *s)
     hdr.sample_rate             = AV_RL32(buf +  8);
     hdr.stereo                  = AV_RL16(buf + 12);
 
+    if (hdr.sample_rate <= 0)
+        return AVERROR_INVALIDDATA;
+
     par                         = st->codecpar;
     par->codec_type             = AVMEDIA_TYPE_AUDIO;
     par->codec_id               = AV_CODEC_ID_ADPCM_IMA_SSI;