Fix getExceptionMessage().

dvmAddTrackedAlloc() doesn't like NULL objects.

Bug 3023981.

Change-Id: I181e4dab841b063d9e2f450292732d808e70aaba
diff --git a/vm/Exception.c b/vm/Exception.c
index 35151f2..ddb84e5 100644
--- a/vm/Exception.c
+++ b/vm/Exception.c
@@ -1261,7 +1261,8 @@
 
         dvmCallMethod(self, getMessageMethod, exception, &result);
         messageStr = (StringObject*) result.l;
-        dvmAddTrackedAlloc((Object*) messageStr, self);
+        if (messageStr != NULL)
+            dvmAddTrackedAlloc((Object*) messageStr, self);
 
         dvmChangeStatus(self, oldStatus);
     } else {