MdePkg: Fix conditionally uninitialized variables

Fixes CodeQL alerts for CWE-457:
https://cwe.mitre.org/data/definitions/457.html



Note that this change affects the actual return value from the
following functions. The functions documented that if an integer
overflow occurred, MAX_UINTN would be returned. They were
implemented to actually return an undefined value from the stack.

This change makes the function follow its description. However, this
is technically different than what callers may have previously
expected.

MdePkg/Library/BaseLib/String.c:
  - StrDecimalToUintn()
  - StrDecimalToUint64()
  - StrHexToUintn()
  - StrHexToUint64()
  - AsciiStrDecimalToUintn()
  - AsciiStrDecimalToUint64()
  - AsciiStrHexToUintn()
  - AsciiStrHexToUint64()

Cc: Erich McMillan <emcmillan@microsoft.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Co-authored-by: Erich McMillan <emcmillan@microsoft.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Oliver Smith-Denny <osd@smith-denny.com>