ospfd: Fix Dereference of Null Pointer during config

ospfd: Fix Dereference of Null Pointer during config

This construct:

struct ospf *ospf = vty->index;
if (!ospf)
  return CMD_SUCCESS;

Is present throughout the entire ospfd code base.  The command:
distance ospf external 255

Is not protected by this construct. I added this construct
to the command and in addition did a quick search to find
any others not protected and to protect them.

Ticket: CM-9725
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Don Slice <dslice@cumulusnetworks.com>