SF: Fix initialization error when starting a traces

Make sure mWhere is always initialized

Test: start and stop sf trace

Fixes: 128851228
Change-Id: I82b8d195a33a50ad28a78392372b7e72479ef9f8
diff --git a/services/surfaceflinger/SurfaceTracing.h b/services/surfaceflinger/SurfaceTracing.h
index a75ac61..9484480 100644
--- a/services/surfaceflinger/SurfaceTracing.h
+++ b/services/surfaceflinger/SurfaceTracing.h
@@ -80,7 +80,7 @@
 
     const SurfaceFlinger& mFlinger;
 
-    char* mWhere;
+    const char* mWhere = "";
     status_t mLastErr = NO_ERROR;
     std::thread mThread;
     std::condition_variable mConditionalVariable;