Fix crash on Windows if loading a plugin fails.

Closes #1866. Thanks to roebotron.

ChangeLog.txt

@@ -62,6 +62,7 @@ Broker:
 - Fix `log_timestamp_format` not applying to `log_dest topic`. Closes #1862.
 - Add the `bridge_max_packet_size` option. Closes #265.
 - Add the `bridge_bind_address` option. Closes #1311.
+- Fix crash on Windows if loading a plugin fails. Closes #1866.
 
 Client library:
 - Client no longer generates random client ids for v3.1.1 clients, these are

src/security.c

@@ -35,7 +35,7 @@ void LIB_ERROR(void)
 {
 #ifdef WIN32
 	char *buf;
-	FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_STRING,
+	FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
 			NULL, GetLastError(), LANG_NEUTRAL, (LPTSTR)&buf, 0, NULL);
 	log__printf(NULL, MOSQ_LOG_ERR, "Load error: %s", buf);
 	LocalFree(buf);