A couple fixes in add_rule() for XFLG_ABS_IF_SLASH:

- Remove the trailing slash earlier, so that it doesn't
  affect the XFLG_ABS_IF_SLASH check.
- Count the slashes earlier so that the XFLG_ABS_IF_SLASH
  can use it instead of using a strchr() all that could
  scan past the end of the input.