Don't log "WAIT_FOR_CONCURRENT_GC blocked 0ms".

Stupid to spend more time logging than we spent waiting. (Highly visible
in the zygote's preloading phase.)

Change-Id: If2d6b6093b3bebae0887aa385745b7ed9225216b
diff --git a/vm/alloc/Heap.cpp b/vm/alloc/Heap.cpp
index 1d06dfe..a68a416 100644
--- a/vm/alloc/Heap.cpp
+++ b/vm/alloc/Heap.cpp
@@ -724,5 +724,7 @@
         dvmChangeStatus(self, oldStatus);
     }
     u4 end = dvmGetRelativeTimeMsec();
-    ALOGD("WAIT_FOR_CONCURRENT_GC blocked %ums", end - start);
+    if (end - start > 0) {
+        ALOGD("WAIT_FOR_CONCURRENT_GC blocked %ums", end - start);
+    }
 }