Fixed the "image sequence" capture not failing when a pattern isn't found.
Fixed the "image sequence" capture not failing when a pattern isn't found.

at can't be a null pointer, so the condition was always false, and
a nonsensical pattern like "image.png%00d" was being inferred.

opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,apavlenko/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv