ShellPkg: Fix Optional Data rewriting with bcfg

[tormodvolden]

Description

When modifying the Optional Data of a boot option with bcfg boot -opt the result was corrupted data, for instance a concatenation of old data, heap contents, and new data. This was due to a erronous calculation of the original optional data length.

In addition to fixing the calculation, add explaining comments and introduce a helper variable, to not abuse other variables and confuse readers (including the author).

Tagging it "security" because the bug can potentially be used for OOB access.

• [ ] Breaking change?
  • Breaking change - Does this PR cause a break in build or boot behavior?
  • Examples: Does it add a new library class or move a module to a different repo.
• [x] Impacts security?
  • Security - Does this PR have a direct security impact?
  • Examples: Crypto algorithm change or buffer overflow fix.
• [ ] Includes tests?
  • Tests - Does this PR include any explicit test code?
  • Examples: Unit tests or integration tests.

How This Was Tested

Before:

Shell> bcfg boot -opt 2 ^"abc^"
Shell> bcfg boot dump -v -b    
  ...
  Optional- Y
  00000000: 61 00 62 00 63 00 00 00-                         *a.b.c...*

Shell> bcfg boot -opt 2 ^"def^"
Shell> bcfg boot dump -v -b    
  ...
  Optional- Y
  00000000: 61 00 62 00 63 00 00 00-70 74 61 6C AF AF AF AF  *a.b.c...ptal....*
  00000010: 64 00 65 00 66 00 00 00-                         *d.e.f...*

After:

Shell> bcfg boot -opt 2 ^"abc^"
Shell> bcfg boot dump -v -b    
  ...
  Optional- Y
  00000000: 61 00 62 00 63 00 00 00-                         *a.b.c...*

Shell> bcfg boot -opt 2 ^"def^"
Shell> bcfg boot dump -v -b    
  ...
  Optional- Y
  00000000: 64 00 65 00 66 00 00 00-                         *d.e.f...*

Integration Instructions

N/A

[pierregondois]

Hello @leiflindholm @mdkinney @ajfish , Just a ping in case the PR looks good to you