Fix segmentation fault during syncing

[halfalicious]

Fix #5832, #5633

I suspect the segmentation fault is occurring when a peer reconnects while its disconnect is
still pending (see here for a detailed description), which results in a live Session but no entry in
EthereumCapability::m_peers.

EthereumCapability::interpretCapabilityPacket directly accesses m_peers via the [] operator rather than using a lookup function, which can cause a segfault if the entry with the node ID has already been removed.

This fix will trigger an exception to be thrown when the peer being looked up is not present in EthereumCapability::m_peers. This isn't ideal, but is good enough for now until we can design a proper fix for the "reconnect during pending disconnect" issue (#5835)

[codecov-io]

Codecov Report

Merging #5834 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5834      +/-   ##
==========================================
- Coverage    64.1%   64.09%   -0.01%     
==========================================
  Files         362      362              
  Lines       30903    30902       -1     
  Branches     3429     3429              
==========================================
- Hits        19811    19808       -3     
+ Misses       9865     9863       -2     
- Partials     1227     1231       +4

[gumb0]

Do I understand correctly that the reason for crash is that new EthereumPeer is default-constructed by operator [] and then it tries to acccess null EthereumPeer::m_host pointer and crashes?

Then could we remove default constructor of EthereumPeer ?

[halfalicious]

Ran a sync overnight on Linux for ~9 hours and didn't hit the AV

[halfalicious]

Do I understand correctly that the reason for crash is that new EthereumPeer is default-constructed by operator [] and then it tries to acccess null EthereumPeer::m_host pointer and crashes?

Then could we remove default constructor of EthereumPeer ?

@gumb0: Yes, that's correct - attempting to retrieve an element from the unordered map which doesn't exist (via the [] operator) results in it being default constructed and inserted, then returned. When we try to deref m_host we get the av.

Good idea about removing the default ctor, I'll do that.

[halfalicious]

Rebased and updated changelog