Fix a crash during the VM shutdown.

The routine to shutdown the threads in a heap source was not checking
to see whether a heap source had been created.  This caused a crash
when VM requested a shutdown before it had fully initialized.

Change-Id: Iea0c7e9ed86ede881986a6576d9f973b2ec8c36d
diff --git a/vm/alloc/HeapSource.c b/vm/alloc/HeapSource.c
index f685fe9..8cf4b44 100644
--- a/vm/alloc/HeapSource.c
+++ b/vm/alloc/HeapSource.c
@@ -588,7 +588,9 @@
 
 void dvmHeapSourceThreadShutdown(void)
 {
-    gcDaemonShutdown();
+    if (gDvm.gcHeap != NULL) {
+        gcDaemonShutdown();
+    }
 }
 
 /*