Seems reproducible:
Start a base defence mission in skirmish. Choose "initial" base variant. No need to modify other settings. Only click turn done a few (maybe 2) times. gdb backtrace:
<source lang="none">
(player 0) It's team 1's turn!
Rerouted for 1 LEs in 0.00s
anonymissimus has taken control over team 1.
Team 1 ended round
Team 7's round started!
Aliens forces have occupied their target zone!
Team 7 ended round
Team 1's round started!
Team 1 ended round
Team 7's round started!
Music: track changed from Crystan-Battlescape03 to PsymongO1.

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffe6301700 (LWP 11249)]
0x00007fffe6343b94 in G_MissionThink (self=0x4bddd30) at src/game/g_mission.cpp:274
274 if (msg[0] == '_')
(gdb) bt
#0 0x00007fffe6343b94 in G_MissionThink (self=0x4bddd30) at src/game/g_mission.cpp:274
#1 0x00007fffe633bb41 in G_EdictsThink () at src/game/g_edicts.cpp:320
#2 0x00007fffe6341da7 in G_RunFrame () at src/game/g_main.cpp:374
#3 0x000000000056dfc0 in SV_RunGameFrame () at src/server/sv_game.cpp:761
#4 0x000000000056df57 in SV_RunGameFrameThread (data=0x0) at src/server/sv_game.cpp:747
#5 0x00007ffff74d4ad8 in ?? () from /usr/lib/x86_64-linux-gnu/libSDL-1.2.so.0
#6 0x00007ffff7514109 in ?? () from /usr/lib/x86_64-linux-gnu/libSDL-1.2.so.0
#7 0x00007ffff69be182 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
#8 0x00007ffff5aaa47d in clone () from /lib/x86_64-linux-gnu/libc.so.6
</source>

(I hope skirmish is sufficient as a testing ground for a base defence during the campaign.)