core: Fixup default_context handling

core: Fixup default_context handling

Now that we set the default context earlier in libusb_init:
-There is no need to do it a second time at the end of libusb_init
-We need to set it to NULL if libusb_init fails later on
-Since it now can be set then reset by libusb_init, libusb_exit needs to
 take the lock while testing if the passed in ctx == default_context

Signed-off-by: Hans de Goede <hdegoede@redhat.com>