Assert that we don't kill tox before killing toxav.

[iphydf]

Possibly this is what's happening in #1149.

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This change is 

[sudden6]

@zoff I don't think it would be a good design if the destructor of an object can fail without it being a programmer error.

Reviewed 1 of 1 files at r1.
Reviewable status: 0 of 1 approvals obtained

[codecov]

Codecov Report

Merging #1154 into master will increase coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1154     +/-   ##
========================================
+ Coverage    82.7%   82.8%   +<.1%     
========================================
  Files          82      82             
  Lines       14667   14668      +1     
========================================
+ Hits        12141   12146      +5     
+ Misses       2526    2522      -4

Impacted Files	Coverage Δ
toxcore/tox.c	65.8% <100%> (ø)	⬆️
auto_tests/toxav_many_test.c	92.5% <0%> (-4.1%)	⬇️
toxcore/LAN_discovery.c	83% <0%> (-1.9%)	⬇️
toxav/toxav.c	68.5% <0%> (+0.3%)	⬆️
toxcore/onion_client.c	95.9% <0%> (+0.6%)	⬆️
toxav/msi.c	66.2% <0%> (+1.6%)	⬆️

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