Force storing the result of crypto_memcmp in the test.

[iphydf]

So we don't accidentally elide the call given that it's a pure function
and its result isn't used.

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This change is 

[robinlinden]

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

[codecov]

Codecov Report

Merging #1061 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1061     +/-   ##
========================================
+ Coverage    82.4%   82.6%   +0.2%     
========================================
  Files          80      80             
  Lines       14317   14318      +1     
========================================
+ Hits        11801   11840     +39     
+ Misses       2516    2478     -38

Impacted Files	Coverage Δ
toxcore/crypto_core_test.cc	90.3% <100%> (+0.3%)	⬆️
toxcore/net_crypto.c	93.3% <0%> (-0.3%)	⬇️
toxcore/Messenger.c	86.2% <0%> (ø)	⬆️
toxav/toxav.c	68.8% <0%> (+1.1%)	⬆️
toxcore/group.c	73.1% <0%> (+1.4%)	⬆️
toxav/msi.c	66.7% <0%> (+2.2%)	⬆️
auto_tests/toxav_many_test.c	96.6% <0%> (+4%)	⬆️
auto_tests/typing_test.c	100% <0%> (+6%)	⬆️
auto_tests/set_name_test.c	100% <0%> (+6.3%)	⬆️

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