Fix using uninitialized mutex on call end

[sphaerophoria]

Closes #1194

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This change is 

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[iphydf]

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

[iphydf]

if (...) {

[sphaerophoria]

Fixed, thanks.

[codecov]

Codecov Report

Merging #1195 into master will increase coverage by 0.3%.
The diff coverage is 25%.

@@           Coverage Diff            @@
##           master   #1195     +/-   ##
========================================
+ Coverage    82.9%   83.2%   +0.3%     
========================================
  Files          82      82             
  Lines       14641   14646      +5     
========================================
+ Hits        12151   12200     +49     
+ Misses       2490    2446     -44

Impacted Files	Coverage Δ
toxav/toxav.c	69.5% <25%> (+0.7%)	⬆️
toxcore/net_crypto.c	94.3% <0%> (-0.1%)	⬇️
toxcore/onion_client.c	95.9% <0%> (ø)	⬆️
toxcore/Messenger.c	86.7% <0%> (ø)	⬆️
toxcore/group.c	76.4% <0%> (+0.3%)	⬆️
auto_tests/network_test.c	100% <0%> (+2.4%)	⬆️
toxcore/TCP_server.c	77.7% <0%> (+2.7%)	⬆️
toxcore/TCP_client.c	67.1% <0%> (+2.7%)	⬆️
toxav/msi.c	68.4% <0%> (+3.3%)	⬆️

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