MdePkg/BaseLib: Fix out-of-bounds reads in SafeString

MdePkg/BaseLib: Fix out-of-bounds reads in SafeString

There was a OOB access in *StrHexTo* functions, when passed strings like
"XDEADBEEF".

OpenCore folks established an ASAN-equipped project to fuzz Ext4Dxe,
which was able to catch these (mostly harmless) issues.

Cc: Vitaly Cheptsov <vit9696@protonmail.com>
Cc: Marvin H?user <mhaeuser@posteo.de>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Acked-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@Intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>