efi: Allow efi_free() to be called with size of 0

Make efi_free() safely callable with size of 0, similar to free() being
callable with NULL pointers, and do nothing in that case.
Remove size checks that this makes redundant.  This also avoids some
size checks in the ARM EFI stub code that will be added as well.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>