bgpd: check for bgp instance before processing interfaces

bgpd: check for bgp instance before processing interfaces

Problem reported with the log displaying error messages if bgpd was
enabled in /etc/frr/daemons but bgp wasn't actually configured.  The
problem was due to operating on interfaces before if_create had been
called (which happens at "router bgp" not at frr starting.  Moved the
checks for the bgp instance before operating on interfaces.  Manual
testing successful and bgp-smoke completed with no new issues.

Ticket: CM-13504
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed-by: CCR-6738