make X509_CRL_METHOD_free() NULL-safe

[Theo Buehler]

This is a practically unused function (both in our tree and globally),
but it is the only remaining public *_free() function in libcrypto that
segfaults on being passed NULL. I'll of course add it to the regress
tests should this go in.

Index: lib/libcrypto/asn1/x_crl.c
===================================================================
RCS file: /var/cvs/src/lib/libcrypto/asn1/x_crl.c,v
retrieving revision 1.30
diff -u -p -r1.30 x_crl.c
--- lib/libcrypto/asn1/x_crl.c  17 Mar 2018 14:33:20 -0000      1.30
+++ lib/libcrypto/asn1/x_crl.c  22 Apr 2018 12:12:14 -0000
@@ -675,7 +675,7 @@ X509_CRL_METHOD_new(int (*crl_init)(X509
 void
 X509_CRL_METHOD_free(X509_CRL_METHOD *m)
 {
-       if (!(m->flags & X509_CRL_METHOD_DYNAMIC))
+       if (m == NULL || !(m->flags & X509_CRL_METHOD_DYNAMIC))
                return;
        free(m);
 }