Prevent 80-test_cmp_http from accidentally killing perl in error.

Prevent 80-test_cmp_http from accidentally killing perl in error.

If there is an issue with setting up the test environment in this test,
pid is not set so stop_server kills the perl process. A guard has been
added to prevent this situation.

Fixes: #22014
Signed-off-by: Randall S. Becker <randall.becker@nexbridge.ca>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22015)