Mp4: fixed potential overflow in ngx_http_mp4_crop_stts_data().

Both "count" and "duration" variables are 32-bit, so their product might
potentially overflow.  It is used to reduce 64-bit start_time variable,
and with very large start_time this can result in incorrect seeking.

Found by Coverity (CID 1499904).

src/http/modules/ngx_http_mp4_module.c

@@ -2331,7 +2331,7 @@ ngx_http_mp4_crop_stts_data(ngx_http_mp4_file_t *mp4,
         }
 
         start_sample += count;
-        start_time -= count * duration;
+        start_time -= (uint64_t) count * duration;
         entries--;
         entry++;
     }