Crash when clicking the board with empty hand

[josephcsible]

In Seuphorica, if you click certain spots on the board while your hand is empty, the game will crash. The problem is here:

hyperrogue/rogueviz/seuphorica.cpp

Line 1132 in 8b7e82b

for(auto& s: snapshots) s.erase(drawn[0].id);

drawn is empty there, so accessing drawn[0] results in a crash.