test/radix/quic_bindings.c: move locking after child_script_info assignment

test/radix/quic_bindings.c: move locking after child_script_info assignment

Coverity complains that some child_script_info field accesses are guarded
by the mutexes, while others are not;  placate it by performing the assignment
before taking the lock.

Resolves: https://scan5.scan.coverity.com/#/project-view/65248/10222?selectedIssue=1665151
References: https://github.com/openssl/project/issues/1362
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28406)