avformat/dhav: Fix second integer overflow in get_duration()

Fixes: ada-2-poc.dhav

Found-by: Claude and Ada Logics. This issue was found by Anthropic from using agents to study security of open source projects, and I am from Ada Logics helping validate the found issues and report to maintainers.
(cherry picked from commit 50e65074f5cca638d6dd4cf9db4b6dcf4f0a863e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>