In retry_transfer_wrapper, do not check url_interrupt_cb, causes problems

when writing and pressing q during encoding. Instead, check url_interrupt_cb
at the end.

Note that when a protocol is interrupted by url_interrupt_cb, some data may
be silently discarded: the protocol context is not suitable for anything
anymore.

Signed-off-by: Nicolas George <nicolas.george@normalesup.org>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>