Pipe global thread lock: release it also on return with error

Fucking goto

src/common/imageio.c

@@ -1162,6 +1162,7 @@ int dt_imageio_export_with_flags(const int32_t imgid, const char *filename,
   return 0; // success
 
 error:
+  dt_pthread_mutex_unlock(&darktable.pipeline_threadsafe);
   dt_dev_pixelpipe_cleanup(&pipe);
 error_early:
   dt_dev_cleanup(&dev);