check the return value of BIO_new_file()

Reviewed-by: NPaul Dale <pauli@openssl.org>
Reviewed-by: NTomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17820)

apps/s_client.c

@@ -1146,6 +1146,10 @@ int s_client_main(int argc, char **argv)
             break;
         case OPT_MSGFILE:
             bio_c_msg = BIO_new_file(opt_arg(), "w");
+            if (bio_c_msg == NULL) {
+                BIO_printf(bio_err, "Error writing file %s\n", opt_arg());
+                goto end;
+            }
             break;
         case OPT_TRACE:
 #ifndef OPENSSL_NO_SSL_TRACE

apps/s_server.c

@@ -1394,6 +1394,10 @@ int s_server_main(int argc, char *argv[])
             break;
         case OPT_MSGFILE:
             bio_s_msg = BIO_new_file(opt_arg(), "w");
+            if (bio_s_msg == NULL) {
+                BIO_printf(bio_err, "Error writing file %s\n", opt_arg());
+                goto end;
+            }
             break;
         case OPT_TRACE:
 #ifndef OPENSSL_NO_SSL_TRACE