MdePkg/BaseMemoryLib*: check for zero length in ZeroMem ()

MdePkg/BaseMemoryLib*: check for zero length in ZeroMem ()

Unlike other string functions in this library, ZeroMem () does not
return early when the length of the input buffer is 0. So add the
same to ZeroMem () as well, for all implementations of BaseMemoryLib
living under MdePkg/

This fixes an issue with the ARM implementation of BaseMemoryLibOPtDxe,
whose InternalMemZeroMem code does not expect a length of 0, and always
writes at least a single byte.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>