avformat/mov: Check requested_sample before using it

I am not sure the case described by coverity is possible
but its more robust checking the argument first

Fixes: CID1598441 Improper use of negative value

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -10486,7 +10486,7 @@ static int mov_seek_stream(AVFormatContext *s, AVStream *st, int64_t timestamp,
         // If we've reached a different sample trying to find a good pts to
         // seek to, give up searching because we'll end up seeking back to
         // sample 0 on every seek.
-        if (!can_seek_to_key_sample(st, requested_sample, next_ts) && sample != requested_sample)
+        if (sample != requested_sample && !can_seek_to_key_sample(st, requested_sample, next_ts))
             break;
 
         timestamp = next_ts;