Remove an NULL ptr deref in an error path

The |passwd| variable in the code can be NULL if it goes to the err label.
Therefore we cannot call strlen on it without first checking that it is non
NULL.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>

ssl/tls_srp.c

@@ -393,6 +393,7 @@ int srp_generate_client_master_secret(SSL *s)
  err:
     BN_clear_free(K);
     BN_clear_free(x);
+    if (passwd != NULL)
         OPENSSL_clear_free(passwd, strlen(passwd));
     BN_clear_free(u);
     return ret;