Fix segfault caused by destruction order of Event and Connection

[azeey]

🦟 Bug fix

Fixes gazebosim/gz-sim#906

Summary

A segfault occurs in the destructor of common::Connection if the common::Event its associated with has already been deleted. This patch clears the event pointer of common::Connection when the associated common::Event is deleted.

Another issue mentioned in gazebosim/gz-sim#906 is that the destructor of std::function crashes if the function it points to is from a shared library that has been unloaded. This is fixed by invoking the destructor as soon as common:EventT::Disconnect is called via common::Connection::~Connection since that's likely to be right before the shared library containing the common::Connection is unloaded.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #234 (d4f7867) into ign-common3 (c6bddd8) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ign-common3     #234      +/-   ##
===============================================
+ Coverage        76.53%   76.55%   +0.01%     
===============================================
  Files               73       73              
  Lines            10379    10386       +7     
===============================================
+ Hits              7944     7951       +7     
  Misses            2435     2435              

Impacted Files	Coverage Δ
events/include/ignition/common/Event.hh	100.00% <100.00%> (ø)

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