mg_schematic: fix leak in lua API, and small cleanup

mg_schematic: fix leak in lua API, and small cleanup

* Fix leak like behaviour if you load multiple schematics in a loop.

* Cleanup check in for, fixing theoretical out of bounds read if
	Schematic::deserializeFromMts reduced the number of elements
	in m_nodenames. A != check may need an overflow of the counter
	before it hits, if origsize is larger than m_nodenames.size().

* Fix function name passed to errorstream: it was wrong. Also use
	__FUNCTION__ instead of manually using the method name at other
	places in the function.

* Don't shadow the name member in the loop.