We try to handle aspect ratios in metadata (e.g. stretching 1440x1080 videos out to 1920x1080 when requested), and FFmpeg will usually return a 1/1 aspect ratio even if it can't determine an aspect ratio. However, as mentioned in the documentation, sometimes it returns a 0/0 aspect ratio when it can't determine an aspect ratio, which we didn't handle and would lead to the code allocating a buffer with a 0px height. This commit handles both 1/1 and 0/0 aspect ratios in accordance with the FFmpeg documentation. Reference: https://www.ffmpeg.org/doxygen/4.0/structAVFrame.html#a62f9c20541a83d37db7072126ff0060d