Perhaps nan/inf protection should be more comprehensive

[sprunk]

a87d53b added nan/inf protection to luaL_checknumber and lua_tofloat.

• what about lua_tonumber? It also produces floats and seems to be used a lot but didn't get protection.
• should functions that produce an int also get protection? In Lua, the number type is a float and the engine does a cast when it wants an int. Isn't this UB if the float was nan/inf? Can it be UB or implementation defined if the float was outside int bounds? This one is less about crashes (since all ints are "valid") and more about desync. I assume it's handled by all of our usual SSE/streflop/etc magic that prevents all the other ways floats could desync but perhaps it's worth checking either way.