EMI: Make Lua_V2::PlayLoadedSound more robust

Don't abort() when EMI's LUA code calls PlayLoadedSound
with nil parameter.
EMI: Avoid +/-inf values for volumes

Ensure that all math operations are done with floating point
arithmetic to avoid a floating point division by zero for small
values of _volume. In this case, the term in parenthesis would
be evaluated first via integer arithmetic and may produce easily 0.