cache: check lseek() return

Fixes CID717496
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/cache.c

@@ -114,7 +114,9 @@ static int64_t cache_seek(URLContext *h, int64_t pos, int whence)
         c->pos= pos;
         return pos;
     }else{
-        lseek(c->fd, c->pos, SEEK_SET);
+        if(lseek(c->fd, c->pos, SEEK_SET) < 0) {
+            av_log(h, AV_LOG_ERROR, "Failure to seek in cache\n");
+        }
         return AVERROR(EPIPE);
     }
 }