MdeMdeModulePkg/UsbBusDxe: Fixed a possible memory leak bug introduced at r14226
The r14226 check-in indeed has memory leak in allocated "Child" pointer. UsbBusDriverBindingStop() may dereference this pointer and may bring exception on invalid memory access

Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>



git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14251 6f19259b-4bc3-4df7-8a09-765794883524