QVM: Fix native progs crash on changelevel.

[dsvensson]

Regression introduced in 0650610.

[dsvensson]

>>> bt
#0  PR_ClearThreads (prinst=0x555555ad8b00 <q1qvmprogfuncs>) at /..../fteqw/engine/common/pr_bgcmd.c:6986
#1  0x000055555574c7dc in PR_Deinit () at /.../fteqw/engine/server/pr_cmds.c:774
#2  0x00005555557829fc in SV_SpawnServer (server=0x7ffffffdb060 "bravado", startspot=0x0, noents=qfalse, usecinematic=qfalse, playerslots=0)
    at /.../fteqw/engine/server/sv_init.c:944
#3  0x00005555557df130 in SV_Map_f () at /.../fteqw/engine/server/sv_ccmds.c:1191
#4  0x0000555555600a86 in Cmd_ExecuteString (text=0x7ffffffdd290 "changelevel \"bravado\"", level=29) at /.../fteqw/engine/common/cmd.c:2989
#5  0x00005555555fa89a in Cbuf_ExecuteLevel (level=29) at /.../fteqw/engine/common/cmd.c:543
#6  0x00005555555fa948 in Cbuf_Execute () at /.../fteqw/engine/common/cmd.c:570
#7  0x0000555555794740 in SV_Frame () at /.../fteqw/engine/server/sv_main.c:5755
#8  0x0000555555926740 in main (argc=7, argv=0x7fffffffd9a8) at /.../fteqw/engine/server/sv_sys_unix.c:1109