Avoid sending group messages to a peer before we have its group number

[zugz]

Hopefully fixes #1285

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This change is 

[codecov]

Codecov Report

Merging #1287 into master will increase coverage by 0.1%.
The diff coverage is 96.7%.

@@           Coverage Diff            @@
##           master   #1287     +/-   ##
========================================
+ Coverage      83%   83.2%   +0.1%     
========================================
  Files          82      82             
  Lines       14957   14960      +3     
========================================
+ Hits        12427   12447     +20     
+ Misses       2530    2513     -17

Impacted Files	Coverage Δ
toxcore/group.c	78.8% <96.7%> (ø)	⬆️
toxcore/friend_connection.c	94.9% <0%> (-1.2%)	⬇️
toxcore/net_crypto.c	94.1% <0%> (-0.9%)	⬇️
toxcore/Messenger.c	87% <0%> (ø)	⬆️
toxcore/DHT.c	77.8% <0%> (ø)	⬆️
toxcore/onion_client.c	96% <0%> (+0.7%)	⬆️
toxcore/TCP_server.c	82.9% <0%> (+2.5%)	⬆️
toxcore/TCP_client.c	67.5% <0%> (+2.8%)	⬆️

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[anthonybilinski]

As commented on #1285, this doesn't seem to fix the issue. Is there anything else meant by the ticket assign?

I won't be able to get back to the environment that I was repoing on, and can't repro locally. My original test was run incorrectly.

Fix seems likely to solve my issue, but I won't be able verify.