Fix internal line editor buffer overflow (heap corruption)

... wasn't accounting for length of the 'top' buffer (when supplied).

Also, for raw input mode:
- flush the input buffer (stray LF) before accepting input
- turn off raw input mode if run out of bytes (max lines/length reached)

I was investigating the reported error by Nelgin:
  !ERROR in writemsg.cpp line 1214 (msgeditor) checking
		"max lines (20) exceeded" access=46
which I was unable to reproduce, when I encountered the issues addressed
in this commit.