[video_player] Fix crash when videoTrack has zero-duration minFrameDuration

[Dennisaurus-Rex]

Fixes a crash on iOS caused by a zero-duration frame duration. When creating the AVMutableVideoComposition a check is made against CMTIME_IS_VALID but this doesn't guard against a frame duration of 0. I just added one more check to make sure the frame duration is > 0.

Fixes flutter/flutter#151031

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Code Review

This pull request addresses a crash in FVPVideoPlayer.m by ensuring that videoTrack.minFrameDuration is greater than zero before assigning it to videoComposition.frameDuration. The reviewer suggested using CMTimeCompare instead of CMTimeGetSeconds to compare CMTime structures, which is more idiomatic and avoids potential precision issues.