fix infinite loop when announcing

[ssiloti]

In torrent::announce_with_tracker there is a check after announcing all of
a tracker's endpoints to see if all required announces have already been sent
according to the tier settings.

	if (std::all_of(listen_socket_states.begin(), listen_socket_states.end()
		, [](announce_state const& s) { return s.done; }))
		break;

Commit ab07ece added a check which can cause listen_socket_states to not be
populated if all endpoints for the first tracker are disabled. This causes the
early break out code above to trigger and prevent any announces from being sent.

The extra check is not in torrent::update_tracker_timer though, so if it thinks there
is at least one endpoint ready to announce it will immediately trigger another
on_tracker_announce, again, and again.

Simple fix is to move the offending check below the code which populates
listen_socket_states so the the latter will execute unconditionally.

[codecov-io]

Codecov Report

Merging #4286 into RC_1_2 will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           RC_1_2    #4286      +/-   ##
==========================================
+ Coverage   74.18%   74.35%   +0.17%     
==========================================
  Files         438      438              
  Lines       60719    60717       -2     
  Branches     8975     8975              
==========================================
+ Hits        45044    45148     +104     
+ Misses      11181    11063     -118     
- Partials     4494     4506      +12

Impacted Files	Coverage Δ
src/torrent.cpp	60% <100%> (-0.13%)	⬇️
src/timestamp_history.cpp	92.59% <0%> (-3.71%)	⬇️
src/kademlia/routing_table.cpp	71.57% <0%> (-3.38%)	⬇️
src/close_reason.cpp	17.77% <0%> (-2.23%)	⬇️
src/http_tracker_connection.cpp	63.32% <0%> (-1.94%)	⬇️
src/alert.cpp	54.36% <0%> (-1.86%)	⬇️
test/web_seed_suite.cpp	89% <0%> (-1.5%)	⬇️
include/libtorrent/alert_types.hpp	54.94% <0%> (-1.1%)	⬇️
src/udp_tracker_connection.cpp	54.74% <0%> (-0.84%)	⬇️
src/choker.cpp	21.95% <0%> (-0.82%)	⬇️
... and 25 more

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[arvidn]

thanks! Sometimes I think about rewriting the whole announce logic to more directly express the logic, and be bit more robust (this is not the first time there's been an infinite loop bug in this code). But there's quite a lot of behavior that would need to preserved, so it's not that easy.