quick fix for crash on the start of line regex

[lifenjoiner]

• the 1st try has set flag boost::match_prev_avail
• the 2 tries does not work for Replace at the same time, currently

Should fix https://github.com/stefankueng/grepWin/issues/405.

I guess there could be a refactoring for CSearchDlg::SearchFile. The code is long, and takes time.