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.

dists/engine-data/patches/emi/residualvm-prefs.lua

@@ -38,6 +38,11 @@ end
 
 --Overwrite stock loadprefs function
 system_prefs.loadprefs = function(table, filename)
+	--avoid installing tag methods repeatedly when starting a new game
+	if system_prefs.defaults_cfg ~= nil then
+		return
+	end
+
 	PrintDebug("Function system_prefs.loadprefs modified for ResidualVM loaded")
 
 	--save old cfg table as default values