Fix nullpointer exception in CompatConfig

The override config parser in CompatConfig will sometimes return a
null config (if the config is empty or malformed).

Bug: 188732264
Test: device boots
Change-Id: Ic52ad9d2e440e5551abb5e8399d97a8ca0fc42a1

services/core/java/com/android/server/compat/CompatConfig.java

@@ -603,6 +603,10 @@ final class CompatConfig {
 
         try (InputStream in = new BufferedInputStream(new FileInputStream(overridesFile))) {
             Overrides overrides = com.android.server.compat.overrides.XmlParser.read(in);
+            if (overrides == null) {
+                Slog.w(TAG, "Parsing " + overridesFile.getPath() + " failed");
+                return;
+            }
             for (ChangeOverrides changeOverrides : overrides.getChangeOverrides()) {
                 long changeId = changeOverrides.getChangeId();
                 CompatChange compatChange = mChanges.get(changeId);