Set constraint, may be caused wrong result in non-UTF8 locales.  Below
fails in a current master.

  $ pattern=$(printf '^x\|\244\263')
  $ printf '\263\244\263\244\n' |
     env LC_ALL=ja_JP.eucJP src/grep "$pattern" && echo FAIL

skip_remains_mb runs in only state 0, but it's wrong.  Set constraint,
may transit to a state besides state 0 after failure.