fix potential double lock of jpeg mutex

src/zm_image.cpp

@@ -1090,14 +1090,16 @@ bool Image::WriteJpeg(const std::string &filename,
                       const int &quality_override,
                       SystemTimePoint timestamp,
                       bool on_blocking_abort) const {
-  // jpeg libs are not thread safe
-  std::unique_lock<std::mutex> lck(jpeg_mutex);
 
   if (config.colour_jpeg_files && (colours == ZM_COLOUR_GRAY8)) {
     Image temp_image(*this);
     temp_image.Colourise(ZM_COLOUR_RGB24, ZM_SUBPIX_ORDER_RGB);
     return temp_image.WriteJpeg(filename, quality_override, timestamp, on_blocking_abort);
   }
+
+  // jpeg libs are not thread safe
+  std::unique_lock<std::mutex> lck(jpeg_mutex);
+
   int quality = quality_override ? quality_override : config.jpeg_file_quality;
 
   jpeg_compress_struct *cinfo = writejpg_ccinfo[quality];