Fixes segfault occurrence in PEM_write()

Checks if header is NULL or not before calling strlen().

CLA: trivial

Fixes #18825

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from openssl/openssl#18865)