efi_loader: CloseProtocol() requires valid agent handle

Return EFI_INVALID_PARAMETER from CloseProtcol() if the agent handle is not
valid.

Return EFI_INVALID_PARAMETER if the optional controller handle is not
valid.

Return immediately from efi_search_obj if the handle is NULL.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>