[openssl/openssl] 3b9082: Fixes segfault occurrence in PEM_write()

[Sahil Siddiq]

  Branch: refs/heads/OpenSSL_1_1_1-stable
  Home:   https://github.com/openssl/openssl
  Commit: 3b9082c844913d3a0efada9fac0bd2924ce1a8f2
      https://github.com/openssl/openssl/commit/3b9082c844913d3a0efada9fac0bd2924ce1a8f2
  Author: valdaarhun <icegambit91 at gmail.com>
  Date:   2022-07-29 (Fri, 29 Jul 2022)

  Changed paths:
    M crypto/pem/pem_lib.c

  Log Message:
  -----------
  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 at openssl.org>
Reviewed-by: Ben Kaduk <kaduk at mit.edu>
Reviewed-by: Paul Dale <pauli at openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18865)

(cherry picked from commit 205957405d08ef199e6ab654e333a627bbca9ccc)