Small fixes in new parser

[Schamper]

Some small fixes I encountered while trying to use the new parser code in dissect.executable.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (0edfc10) to head (39f24c3).

Files with missing lines	Patch %	Lines
dissect/cstruct/parser.py	0.00%	16 Missing ⚠️
dissect/cstruct/lexer.py	0.00%	13 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #164   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         22      22           
  Lines       2700    2719   +19     
=====================================
- Misses      2700    2719   +19     

Flag	Coverage Δ
unittests	0.00% <0.00%> (ø)

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[codspeed]

Merging this PR will not alter performance

✅ 14 untouched benchmarks

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_{Comparing parser-fixes (39f24c3) with main (0edfc10)}