fix: handle integer suffix in #line directive without crashing

[SAY-5]

A #line directive with an integer suffix (e.g. #line 0u) caused an unhandled ValueError: invalid literal for int() with base 10: '0u' in c_lexer.py. The _decimal_constant regex already accepts suffixes like u/U/l/L, so a fuzz-generated input with a suffixed line number reaches int(pp_line) and crashes.

This wraps the int() call in a try/except ValueError and reports a proper "invalid #line directive" parse error instead. Three tests added to TestCLexerErrors.test_preprocessor covering 0u, 1U, and 10uLL.

Closes #587

[eliben]

Thanks for the contribution!