libfdk-aacdec: Fix a boundary check

This avoids potential out of bounds writes, with potential future
versions of the library.

Bug-Id: CID 1254945
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
dashenc: Change the duration fields to 64 bit

For the last_duration field, it's mostly theoretical, but the
total_duration field more probably may need to actually be 64 bit.

Bug-Id: CID 1254944
Signed-off-by: Martin Storsjö <martin@martin.st>