uloop: fix deleting pending fd events on uloop_fd_del

uloop: fix deleting pending fd events on uloop_fd_del

When a fd gets deleted internally due to errors, fd->registered gets set
to false before events are moved to the staging array.
This leads to pending events not getting cleared properly when the fd
user finally calls uloop_fd_del.
Fix this by moving the check down and always checking for pending
events.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>