Restore vector type check

[OgelGames]

Restores the type check for vectors that was lost in b38ffde.

Example error messages, without and with the type check:

ERROR[Main]: ServerError: AsyncErr: Lua: Runtime error from mod 'test' in callback ScriptApiEnv::environment_Step(): ...\builtin\common\vector.lua:380: attempt to index local 'v' (a nil value)
ERROR[Main]: stack traceback:
ERROR[Main]:       ...\builtin\common\vector.lua:380: in function <...\builtin\common\vector.lua:379>
ERROR[Main]:       [C]: in function 'set_pos'
ERROR[Main]:       ...\mods\test\init.lua:3: in function 'func'

ERROR[Main]: ServerError: AsyncErr: Lua: Runtime error from mod 'test' in callback ScriptApiEnv::environment_Step(): "Invalid vector (expected table got nil)."
ERROR[Main]: stack traceback:
ERROR[Main]:       [C]: in function 'set_pos'
ERROR[Main]:       ...\mods\test\init.lua:3: in function 'func'

[sfan5]

I think we established that table operations are faster from Lua than from C++, so this check will have been intentionally left off.

[OgelGames]

But it only checks if the vector is a table, there's are no table operations.

Even if it was intentional I think it should be added back, because it makes the error message clearer.

[sfan5]

I guess.

[SmallJoker]

LGTM