EMI: Fix crash when restarting the game

ResidualVM's EMI patch installs a custom version of system_prefs.loadprefs
which maps config reads to GetResidualVMPreference() calls. The original
system_prefs.cfg is saved and used as source for default values. When
the custom loadprefs function is called a second time (i.e. when using
the New Game option in the main menu) any config read results in an
endless recursion.