ospfd: fix issue with the "no segment-routing prefix A.B.C.D/M" command

ospfd: fix issue with the "no segment-routing prefix A.B.C.D/M" command

Add a missing check to bail out earlier when SR is not configured. The
same command without the "no" prefix has the same check as it prevents
unexpected things (i.e. crashes) from happening.

Fixes the following segfaults:
ospfd aborted: vtysh -c "configure terminal" -c "router ospf" -c "no segment-routing prefix 1.1.1.1/32"
ospfd aborted: vtysh -c "configure terminal" -c "router ospf" -c "no segment-routing prefix 1.1.1.1/32 index 65535 no-php-flag"

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>