fixes bug 297973 "It's possible to allocate a nsAString in the Gecko SDK" r=bryner sr=dbaron a=asa

xpcom/string/public/nsStringAPI.h

@@ -923,6 +923,10 @@ public:
 
 #endif // MOZILLA_INTERNAL_API
 
+protected:
+  // Prevent people from allocating a nsAString directly.
+  ~nsAString_external() {}
+
 private:
   void *v;
 };
@@ -1024,6 +1028,10 @@ public:
 
 #endif // MOZILLA_INTERNAL_API
 
+protected:
+  // Prevent people from allocating a nsACString directly.
+  ~nsACString_external() {}
+
 private:
   void *v;
 };