Fix crash when all durations are zero

[takke]

Hi,

I am developing an Android application for multiple SNS like bluesky, mastodon and misskey.

We have been using this library as an APNG decoder for a social networking service called Misskey, but there was a problem with crashes when loading an incorrect file with a total duration of 0.

This fix prevents crashes as a minimum fix.

It is desirable to be able to set some kind of duration and display all frames as animated, but I could not figure out how to do that, so the first frame is displayed as a fixed frame.

Thanks.

sample:

[CLAassistant]

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