ssl/ssl_lib.c: Avoid crash when SSL_CONNECTION is NULL

ssl/ssl_lib.c: Avoid crash when SSL_CONNECTION is NULL

Detection for sc == NULL is performed after sc is used. Add the
check to the correct place.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/27241)

(cherry picked from commit 7f6cc862c69800a72c49fcfe89e2931ee4ca2e7a)