Ignore "unused-result" warning in super_donators code.

[iphydf]

Also fix some warnings appearing under NDEBUG.

---

This change is 

[codecov]

Codecov Report

Merging #1147 into master will decrease coverage by 0.4%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1147     +/-   ##
========================================
- Coverage    83.2%   82.7%   -0.5%     
========================================
  Files          82      82             
  Lines       14593   14594      +1     
========================================
- Hits        12149   12083     -66     
- Misses       2444    2511     +67

Impacted Files	Coverage Δ
toxcore/DHT.c	76.8% <ø> (ø)	⬆️
toxcore/tox.c	65.7% <100%> (ø)	⬆️
auto_tests/network_test.c	90.3% <0%> (-9.7%)	⬇️
toxav/msi.c	63.6% <0%> (-3.1%)	⬇️
toxcore/TCP_client.c	64.3% <0%> (-2.8%)	⬇️
toxcore/TCP_server.c	75% <0%> (-2.8%)	⬇️
auto_tests/toxav_basic_test.c	81.6% <0%> (-2%)	⬇️
toxav/toxav.c	68.1% <0%> (-1.8%)	⬇️
toxcore/net_crypto.c	93.5% <0%> (-1.6%)	⬇️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1b23222...f59e6ff. Read the comment docs.