Fix gdbreplay checksum calculation

Fix gdbreplay checksum calculation

I needed to use gdbreplay today.  It didn't work quite right, and
using "set debug remote 1" showed that gdb was rejecting some
responses like:

  [remote] Sending packet: $vCont?#49
  [remote] Junk: #vCont?
  [remote] Junk: 8vCont?
  [remote] Junk: 3vCont?
  [remote] Received Ack

The checksum recalculation seems to have gone wrong.  Looking at the
code, it seems like 'where_csum' is calculated inconsistently: in the
main loop it is after the '#' but in the "== 0" case it is before the
'#'.

This patch fixes the problem and also avoids a string copy.

CC: Alexandra Hájková <ahajkova@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>