efi/libstub: Zero initialize heap allocated struct screen_info

After calling uefi interface allocate_pool to apply for memory, we
should clear 0 to prevent the possibility of using random values.

Signed-off-by: Qiang Ma <maqianga@uniontech.com>
Cc: <stable@vger.kernel.org> # v6.6+
Fixes: 732ea9db ("efi: libstub: Move screen_info handling to common code")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>