avformat/nsvdec: Check asize for PCM

Fixes: CID1604527 Overflowed constant

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e83e2465

)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/nsvdec.c

@@ -603,7 +603,7 @@ null_chunk_retry:
         pkt = &nsv->ahead[NSV_ST_AUDIO];
         /* read raw audio specific header on the first audio chunk... */
         /* on ALL audio chunks ?? seems so! */
-        if (asize && st[NSV_ST_AUDIO]->codecpar->codec_tag == MKTAG('P', 'C', 'M', ' ')/* && fill_header*/) {
+        if (asize >= 4 && st[NSV_ST_AUDIO]->codecpar->codec_tag == MKTAG('P', 'C', 'M', ' ')/* && fill_header*/) {
             uint8_t bps;
             uint8_t channels;
             uint16_t samplerate;