"Save Buffers" fix: when you ESC from rendering, it now saves empty

tiles in the file, that read back fast as black. This also solves
crashes on partial written files, when trying to read them.

source/blender/imbuf/intern/openexr/openexr_api.cpp

@@ -526,6 +526,11 @@ void IMB_exr_set_channel(void *handle, char *layname, char *passname, int xstrid
 		printf("IMB_exrtile_set_channel error %s\n", name);
 }
 
+void IMB_exrtile_clear_channels(void *handle)
+{
+	ExrHandle *data= (ExrHandle *)handle;
+	BLI_freelistN(&data->channels);
+}
 
 void IMB_exrtile_write_channels(void *handle, int partx, int party)
 {