Run UBSAN (undefined behaviour sanitizer) on Circle CI.

[iphydf]

This runs in the same build as asan, so "asan" now stands for both asan
and ubsan.

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This change is 

[zoff99]

Reviewable status: complete! 1 of 1 approvals obtained

[codecov]

Codecov Report

Merging #1196 into master will increase coverage by 0.2%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1196     +/-   ##
========================================
+ Coverage    82.7%   82.9%   +0.2%     
========================================
  Files          82      82             
  Lines       14646   14646             
========================================
+ Hits        12119   12154     +35     
+ Misses       2527    2492     -35

Impacted Files	Coverage Δ
toxcore/onion_client.c	95.8% <0%> (-0.2%)	⬇️
toxcore/Messenger.c	86.5% <0%> (-0.1%)	⬇️
toxcore/net_crypto.c	94.2% <0%> (+0.6%)	⬆️
toxcore/LAN_discovery.c	85.8% <0%> (+0.9%)	⬆️
toxav/toxav.c	69% <0%> (+1.9%)	⬆️
toxav/msi.c	66.7% <0%> (+3%)	⬆️
auto_tests/network_test.c	97.5% <0%> (+7.2%)	⬆️

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