Fix segfault when save the render with Multilayer format.

The RE_WriteResult function don't call BLI_make_existing_file
like BKE_write_ibuf, so if you append a directory to the output
path and it don't exist, the exr_begin_write fail to create
the file and blender crash.

Note that this is only 1 line of code and don't touch
anything in the render code.

source/blender/render/intern/source/pipeline.c

@@ -749,6 +749,8 @@ void RE_WriteRenderResult(RenderResult *rr, char *filename, int compress)
 	RenderLayer *rl;
 	RenderPass *rpass;
 	void *exrhandle= IMB_exr_get_handle();
+
+	BLI_make_existing_file(filename);
 	
 	/* composite result */
 	if(rr->rectf) {