gdb: print the SwNodeIndex properly if there are multiple blocks

Change-Id: I178d365e71ef3377b1a2cfc115bb297640161efe
(cherry picked from commit 4f211a0d0c4ef0c248fd3d4493994c1a122c2eb9)
Reviewed-on: https://gerrit.libreoffice.org/11047
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>