games/warmux: Fix build with Clang 6

interface/weapon_menu.cpp:394:12: error: cannot initialize return object of type 'Weapon *' with an rvalue of type 'bool'

return false;
       ^~~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p475478_s336801/logs/errors/warmux-11.04.1_9.log

map/tile.cpp:676:30: error: non-constant-expression cannot be narrowed from type

'uint' (aka 'unsigned int') to 'uint16_t' (aka 'unsigned short') in initializer
list [-Wc++11-narrowing]
SynchTileInfo info = { i, t->GetSynchsum() };
                       ^