Fix parser crashes on transient null lines and temporary string lifetime

[exkrexpexfex]

This PR addresses a series of ASan-reported crashes in the colorer parser and related string handling when testing quick colored popups creation and destruction in scope of elfmz/far2l#3248

Findings

• UnicodeString::UnicodeString(int) built a CString from std::to_string(no).c_str() where the std::string temporary died before CString was consumed, causing a stack-use-after-scope read.
• TextParser::Impl::colorize() threw an exception when LineSource::getLine() returned null, which propagated across threads and left the UI in a bad state.
• After the null-return fix, the parser could still reuse a stale str pointer on a later pass (because clearLine blocked a fresh getLine()), leading to a null/invalid deref at str->length().

Changes

• Make the std::to_string() result live long enough in UnicodeString::UnicodeString(int) by storing it in a local std::string before constructing CString.
• In TextParser::Impl::colorize(), treat null lines as a graceful stop condition instead of throwing; return cleanly.
• Reset str and clearLine when getLine() returns null to avoid reusing stale pointers on subsequent passes.

Notes

• These changes target shutdown/transient editor states where lines are unavailable.
• No behavior change for normal parsing flow; only defensive handling for unavailable lines and invalid lifetimes.

Testing

• Reproduced crash with ASan before; retested after changes and the issue no longer reproduced.