fix potential double lock of jpeg mutex

src/zm_image.cpp

@@ -1092,13 +1092,15 @@ bool Image::WriteJpeg(const char *filename, int quality_override, struct timeval
 }
 
 bool Image::WriteJpeg(const char *filename, int quality_override, struct timeval 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;
 
   struct jpeg_compress_struct *cinfo = writejpg_ccinfo[quality];