BUG/MEDIUM: proxy: fix crash with stop_proxy() called during init

BUG/MEDIUM: proxy: fix crash with stop_proxy() called during init

Willy reported that the following config would segfault right after the
"removing incomplete section 'peer' is emitted:

  peers peers
	bind :2300
  	server n10 127.0.0.1:2310

  listen dummy
  bind localhost:9999

This is caused by the fact that stop_proxy(), which tries to read shared
counters, is called during early init while shared counters are not yet
initialized. To fix the crash, let's check if we're still during starting
phase, in which case we assume the counters are not initialized and we
assume 0 value instead.

No backport needed unless 16eb0fab31 ("MAJOR: counters: dispatch counters
over thread groups") is.