Avoid crashes due to null SettingsSections

If a SettingsFile had at least one section, it was assumed all sections
were correctly filled out. This caused crashes when opening the settings
menus if that was not the case - for example the GFX.ini settings empty
sections are removed by the main dolphin app, putting the .ini file in a
state that would crash the settings window if at least one setting was
changed in it from the default, some sections were left as default.

This adds a subclass of HashMap<String, SettingSection> that constructs a
new SettingSection instead of returning 'null' if the key isn't found,
so the mSettings.get(FILE).get(SECTION).get(SETTING) pattern can be
safely used.

Source/Android/app/src/main/java/org/dolphinemu/dolphinemu/utils/SettingsFile.java

@@ -22,6 +22,32 @@ import java.util.HashMap;
 import java.util.Set;
 import java.util.TreeSet;
 
+/**
+ * A HashMap<String, SettingSection> that constructs a new SettingSection instead of returning null
+ * when getting a key not already in the map
+ */
+final class SettingsSectionMap extends HashMap<String, SettingSection>
+{
+	@Override
+	public SettingSection get(Object key)
+	{
+		if (!(key instanceof String))
+		{
+			return null;
+		}
+
+		String stringKey = (String)key;
+
+		if (!super.containsKey(stringKey))
+		{
+			SettingSection section = new SettingSection(stringKey);
+			super.put(stringKey, section);
+			return section;
+		}
+		return super.get(key);
+	}
+}
+
 /**
  * Contains static methods for interacting with .ini files in which settings are stored.
  */
@@ -256,7 +282,7 @@ public final class SettingsFile
 	 */
 	public static HashMap<String, SettingSection> readFile(final String fileName, SettingsActivityView view)
 	{
-		HashMap<String, SettingSection> sections = new HashMap<>();
+		HashMap<String, SettingSection> sections = new SettingsSectionMap();
 
 		File ini = getSettingsFile(fileName);