core: abandon sync transfers after libusb_close()

This commit adds an additional check to synchronous transfer
progression to prevent accessing a potentially deleted context. This
access can occur if a code calls libusb_close() then libusb_exit()
from one thread while another thread is in a synchronous transfer.

Note that this is likely a user error. The application should wait
for all transfers to complete before calling libusb_close(). This
commit is being offered as a best-effort attempt to prevent the
invalid access and return a useful error to the application.

There may still be a window where an invalid access is possible.
This commit is a work around until a better fix is available.

References #610

Signed-off-by: Nathan Hjelm <hjelmn@google.com>