core: Copy transfer timeout to local variable before using

core: Copy transfer timeout to local variable before using

The flying_transfers lock is taken in libusb_get_next_timeout()
to find the transfer with the next timeout, but the transfer was
being used outside of the lock, resulting in the possibility that
the transfer had been removed from the list and freed at the time
it was used.

Issue reported by Michel Sanches.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>