Problem: Crash (abort/sigsegv) when dealing with single participant in elections

[Mathsoum]

Previous PR about single participant in elections did not clean the group structure properly.
This was causing crashes in specific situations when there have been an election, then one peer was left alone in this election. The structure was not clean and thus when some other peer left, when read garbage memory.

Solution: Clean leader/election attributes of zyre_group when dealing with single participant in elections.

NOTE:
5d15a8b First commit cleans up the attributes.
908553a Second commit change some ifs to avoid checking the same condition multiple times.
1f163ef Bump the patch version to track bugfix

[bluca]

Why is this changing the version? Please remove that

[Mathsoum]

I thought PATCH version numbers were used to notify a bugfix.
Why wouldn't we increase the version in this case ?

[bluca]

breaks windows build:

C:\projects\zyre\src\zyre_node.c(947,54): error C2664: 'void zyre_group_set_leader(zyre_group_t *,zyre_peer_t *)': cannot convert argument 2 from 'zyre_node_t *' to 'zyre_peer_t *' [C:\projects\zyre\build\zyre.vcxproj]
C:\projects\zyre\src\zyre_node.c(947,50): message : Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast [C:\projects\zyre\build\zyre.vcxproj]
C:\projects\zyre\src\zyre_group.h(73,5): message : see declaration of 'zyre_group_set_leader' [C:\projects\zyre\build\zyre.vcxproj]
C:\projects\zyre\src\zyre_node.c(1239,58): error C2664: 'void zyre_group_set_leader(zyre_group_t *,zyre_peer_t *)': cannot convert argument 2 from 'zyre_node_t *' to 'zyre_peer_t *' [C:\projects\zyre\build\zyre.vcxproj]
C:\projects\zyre\src\zyre_node.c([1239](https://ci.appveyor.com/project/zeromq/zyre/builds/48163333/job/wa49rytidt6epykv#L1239),54): message : Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast [C:\projects\zyre\build\zyre.vcxproj]
C:\projects\zyre\src\zyre_group.h(73,5): message : see declaration of 'zyre_group_set_leader'

[Mathsoum]

My mistake.
I though I could set the current peer as the leader of an election where it's the only one present but actually, with a single peer there are no election and therefore no leader.
It is now set to NULL in that case and will be changed during the next election.