Fix NPE when public API removeProximityAlert() used before addProximityAlert().

Bug: 6313992
Change-Id: I905ad9ea771286727ce4a3a2334f2a0dac967c3d

Author: Nick Pelly

services/java/com/android/server/LocationManagerService.java

@@ -1702,7 +1702,9 @@ private void removeProximityAlertLocked(PendingIntent intent) {
 
         mProximityAlerts.remove(intent);
         if (mProximityAlerts.size() == 0) {
-            removeUpdatesLocked(mProximityReceiver);
+            if (mProximityReceiver != null) {
+                removeUpdatesLocked(mProximityReceiver);
+            }
             mProximityReceiver = null;
             mProximityListener = null;
         }