Fix use-after-free in unittest setup code.

When setting the root prefix in a unittest, we need to reset it after
so it isn't accidentally used by other tests.

Bug: chromium:547127
Test: USE="clang asan" FEATURES=test emerge-link update_engine

Change-Id: Ib84bbf265a380976407100a4fba4d2600d62dde6
diff --git a/omaha_request_params.h b/omaha_request_params.h
index 131a8f3..b6b6f04 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -95,7 +95,7 @@
     mutable_image_props_.is_powerwash_allowed = false;
   }
 
-  virtual ~OmahaRequestParams() = default;
+  virtual ~OmahaRequestParams();
 
   // Setters and getters for the various properties.
   inline std::string os_platform() const { return os_platform_; }