MdePkg/BaseLib: rewrite Base64Decode()

MdePkg/BaseLib: rewrite Base64Decode()

Rewrite Base64Decode() from scratch, due to reasons listed in the second
reference below.

Implement Base64Decode() according to the specification added in the
previous patch. The decoder scans the input buffer once, it has no inner
loop(s), and it spills each output byte as soon as the output byte is
complete.

The intent is to only strengthen the checks (sanity and input) relative to
the previous implementation, hence the MAX_ADDRESS checks are reinstated.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Marvin Häuser <mhaeuser@outlook.de>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1891
Ref: http://mid.mail-archive.com/c495bd0b-ea4d-7206-8a4f-a7149760d19a@redhat.com
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Tested-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
[lersek@redhat.com: add last para to commit msg per talks w/ Marvin & Phil]