MdeModulePkg/DxeMain: Zero ImageContext local variable before passing it down to PeCoffLoaderRelocateImageExtraAction()

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Andrew Fish <afish@apple.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@15587 6f19259b-4bc3-4df7-8a09-765794883524