Fix integer overflow for audio packet duration

The result is assigned to a long (64-bit signed integer), but the
intermediate multiplication was stored in an int (32-bit signed
integer).

This value is only used as a fallback when no timestamp could be
retrieved, that's why it did not cause too much harm so far.

Fixes #4536 <https://github.com/Genymobile/scrcpy/issues/4536>