gallium/radeon: Initialize pipe_resource::next to NULL

Fixes lots of piglit tests crashing due to using uninitialized memory.

Fixes: ecd6fce261 ("mesa/st: support lowering multi-planar YUV")
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/drivers/radeon/r600_buffer_common.c

@@ -511,6 +511,7 @@ r600_alloc_buffer_struct(struct pipe_screen *screen,
 	rbuffer = MALLOC_STRUCT(r600_resource);
 
 	rbuffer->b.b = *templ;
+	rbuffer->b.b.next = NULL;
 	pipe_reference_init(&rbuffer->b.b.reference, 1);
 	rbuffer->b.b.screen = screen;
 	rbuffer->b.vtbl = &r600_buffer_vtbl;

src/gallium/drivers/radeon/r600_texture.c

@@ -1040,6 +1040,7 @@ r600_texture_create_object(struct pipe_screen *screen,
 
 	resource = &rtex->resource;
 	resource->b.b = *base;
+	resource->b.b.next = NULL;
 	resource->b.vtbl = &r600_texture_vtbl;
 	pipe_reference_init(&resource->b.b.reference, 1);
 	resource->b.b.screen = screen;