GDBusMethodInvocation: leak and potential crash

GDBusMethodInvocation: leak and potential crash

_g_dbus_method_invocation_new is said to allow method_info == NULL,
but will crash inside g_dbus_method_info_ref when the method_info
really is NULL, because g_dbus_method_info_ref does not allow NULL as
parameter. Fixed by checking for NULL in _g_dbus_method_invocation_new
itself.

The leak itself happens because _g_dbus_method_invocation_new stores a
new reference to the method_info without also unreferencing it. Fixed
by adding the missing unref, protected by an if because the pointer
may be NULL.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=695376