asan: mmo: NULL dereferenc in mmo_xore_32

asan: mmo: NULL dereferenc in mmo_xore_32

mmo_get_loc can return NULL.  It's commented even, and that the caller
then must handle a split field.  mmo_xore_* don't handle split fields,
instead just segfault.  Stop that happening, and refuse to recognise
fuzzed mmo files that trigger this problem.

	* mmo.c (mmo_get_loc): Don't declare inline.
	(mmo_xore_64, mmo_xore_32, mmo_xore_16): Remove forward decls.
	Return pointer, don't dereference NULL.
	(mmo_scan): Return error on mmo_get_loc returning NULL.