[Fix] upstream: add release() for non-success/failure paths

[Fix] upstream: add release() for non-success/failure paths

The new inflight counter introduced for P2C exposed several pre-existing
leaks where a get_* selection had no matching ok()/fail() call. ok() was
unsuitable as a generic retire because it also clears the error count.

Add rspamd_upstream_release() — decrement inflight without touching
errors, latency, or watchers — and apply at four call sites:

- rspamd_proxy.c mirror loop: copy_msg failure after upstream selection
- rspamd_proxy.c master loop: copy_msg failure after upstream selection
- fuzzy_check.c PING: fire-and-forget address lookup
- http_connection.c proxy: hand-off path where new_common drops the
  upstream pointer (per-request tracking left for a follow-up)

Two more leak classes remain for separate PRs: Lua-side retire fallback
via __gc, and librdns retransmit/select pairing in dns.c.

Tests: 9 P2C cases (was 7; +2 covering release behaviour and null safety).