Merge db_input.c:1.39 from HEAD to RELENG_6:

Merge db_input.c:1.39 from HEAD to RELENG_6:

  Reserve two bytes at the end of the DDB input line in db_readline() to
  hold the newline and nul terminator.  Otherwise, there are cases where
  garbage may end up in the command history due to a lack of a nul
  terminator, or input may end up without room for a newline.

  PR:             119079
  Submitted by:   Michael Plass <mfp49_freebsd@plass-family.net>