Fix process leak with make-network-process

Fix process leak with make-network-process

This problem was introduced by the recent async changes (Bug#23808).
* src/process.c (Fmake_process): Move USE_SAFE_ALLOCA later,
so that it follows the start_process_unwind unwind-protect.
Set pid to -1 while the process is being created.
(start_process_unwind): Omit unnecessary emacs_abort test.
(connect_network_socket): Simplify use of counts.  Unwind
bind_polling_period a bit earlier, so that a remove_process
unwind-protect can be added when needed; this is the heart of
the fix.  Undo the unwind-protect just before returning.