[KERNEL32_VISTA] Fix a bug I've introduced while merging PR #7982

dll/win32/kernel32/kernel32_vista/vista.c

···
         return FALSE;
     }
 
-    dwBufferSize = (*lpdwSize + 1) * sizeof(WCHAR);
+    dwBufferSize = sizeof(UNICODE_STRING) + *lpdwSize * sizeof(WCHAR);
     pBuffer = (PUNICODE_STRING)RtlAllocateHeap(RtlGetProcessHeap(), 0, dwBufferSize);
     if (!pBuffer)
     {