Fix null cursor on lastmessage and prevent a double cursor close in same func

src/main/java/fr/unix_experience/owncloud_sms/engine/SmsFetcher.java

@@ -126,6 +126,9 @@ public class SmsFetcher {
 		
 		// Fetch Sent SMS Message from Built-in Content Provider
 		Cursor c = (new SmsDataProvider(_context)).query(mbURI);
+        if (c == null) {
+            return null;
+        }
 		
 		c.moveToNext();
 		
@@ -170,7 +173,6 @@ public class SmsFetcher {
 			results.put(entry);
 		} catch (JSONException e) {
 			Log.e(SmsFetcher.TAG, "JSON Exception when reading SMS Mailbox", e);
-			c.close();
 		}
 		
 		c.close();