[9.20] chg: nil: Fix INSIST copy-paste error checking RADIX_V4 instead of RADIX_V6

The INSIST in isc_radix_insert() checks node->data[RADIX_V4] and
node->node_num[RADIX_V4] twice due to a copy-paste error, never
verifying the RADIX_V6 fields.

Fix the second pair to check RADIX_V6.

Backport of MR !11664

Merge branch 'backport-ondrej/fix-copy-paste-error-checking-RADIX_V4-instead-of-RADIX_V6-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11686