front: Fixed unnavigable path preference

On first install, if the user did not have the folder [sdcard]/roms/n64 on
their device, then they were unable to navigate the path preference to
select a ROM.

src/paulscode/android/mupen64plusae/persistent/PathPreference.java

@@ -113,6 +113,8 @@ protected Object onGetDefaultValue( TypedArray a, int index )
             // Ensure the parent directories exist if requested
             if( forceParentDirs )
                 ( new File( value ) ).mkdirs();
+            else if( !new File( value ).exists() )
+            	value = STORAGE_DIR;
         }
 
         return value;