CXX: Avoid assert with broken input in debug builds

[pragmaware]

Fixes #2884

[codecov]

Codecov Report

Merging #3056 (616156f) into master (cab9828) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3056   +/-   ##
=======================================
  Coverage   87.33%   87.33%           
=======================================
  Files         199      199           
  Lines       47611    47611           
=======================================
  Hits        41579    41579           
  Misses       6032     6032           

Impacted Files	Coverage Δ
parsers/cxx/cxx_parser_tokenizer.c	92.45% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cab9828...616156f. Read the comment docs.

[masatake]

Thank you!