test/radix/terp.c: avoid accessing uninitialised terp on error

test/radix/terp.c: avoid accessing uninitialised terp on error

Stats printing in TERP_run() assumes that terp has been initialised,
which is not the case when a jump to the err label has been performed
before successful TERP_init() call;  avoid it by emplacing it
within a have_terp guard check.

Fixes: 4a2d5fe812f8 "QUIC RADIX: Add RADIX test framework implementation"
Resolves: https://scan5.scan.coverity.com/#/project-view/65248/10222?selectedIssue=1665430
References: https://github.com/openssl/project/issues/1432
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28546)