client: Refactor wl_proxy_destroy critical section

Split wl_proxy_destroy into two pieces, wl_proxy_destroy_unlocked which
performs the critical section code with no locking, and wl_proxy_destroy
which locks before calling that.

We'll use the new unlocked variant later in code that already holds the
lock.

There is a slight functional change - an aborting check is now called
with the lock held. This should be harmless as wl_abort() performs
no locking.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>