Bug 1000079 - Don't let Windows overwrite length 0 strings. r=bsmedberg

toolkit/xre/nsXREDirProvider.cpp

@@ -931,8 +931,8 @@ GetRegWindowsAppDataFolder(bool aLocal, nsAString& _retval)
     return NS_ERROR_NOT_AVAILABLE;
   }
 
-  // |size| includes room for the terminating null character
-  DWORD resultLen = size / 2 - 1;
+  // |size| may or may not include room for the terminating null character
+  DWORD resultLen = size / 2;
 
   _retval.SetLength(resultLen);
   nsAString::iterator begin;
@@ -951,6 +951,11 @@ GetRegWindowsAppDataFolder(bool aLocal, nsAString& _retval)
     return NS_ERROR_NOT_AVAILABLE;
   }
 
+  if (!_retval.CharAt(resultLen - 1)) {
+    // It was already null terminated.
+    _retval.Truncate(resultLen - 1);
+  }
+
   return NS_OK;
 }