efi_loader: out of memory in efi_add_memory_map_pg

Handle out of memory situation in efi_add_memory_map_pg().

Fixes: 5d00995c ("efi_loader: Implement memory allocation and map")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>