FIX crash when `bind'ing over existing binding!..

FIX crash when `bind'ing over existing binding!..

Oh, now that we can redefine macros over i did

  ?0!0[#/var/spool/mail/steffen]? source /home/steffen/.s-mailrc
  Crash NYD listing written to /tmp/s-nail.dat
  Segmentation fault (core dumped)

that was

  s-nail: MACRO 40 bytes <if t && [ "$features" =@ +key-bindings ]>
  s-nail: memreset: freed 2 chunks/8310 bytes
  s-nail: MACRO 36 bytes <bind base $'\e',d mle-snarf-word-fwd>
  Crash NYD listing written to /tmp/s-nail.dat
  Segmentation fault (core dumped)

Uff!  Two occurrences of array accesses did not mask their index --
we carry index and flags in one member --, which didn't seem to
matter that much when i have implemented the stuff, but does
today.