bgpd: debug buffers cleanup and optimization

bgpd: debug buffers cleanup and optimization

Just the first change pushes bgp_update_receive() from 6th to ~14th on a
full internet table load profiling session.

* bgp_debug.c: (bgp_update_receive) The attrstr initialization is expensive,
  moved under the debug conditional where it is used and just initialize the
  first char to NULL.
  (bgp_update_default_send) Initialize attrstr needed for bgp_dump_attr().
  Moved some buffers used for printing IP[4|6] addresses under the debug
  conditionals that use them and reduced its size.

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>