radeonsi: silence a Coverity warning

Coverity doesn't understand that we'll never pass non-NULL for vertex
shaders.

This is a bit lame, actually. A straightforward cross-procedural analysis
limited to this source file should be enough to prove that there's no
NULL-pointer dereference. Oh well.

CID: 1405999
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/drivers/radeonsi/si_debug.c

@@ -478,6 +478,8 @@ static void si_dump_descriptors(struct si_context *sctx,
 	};
 
 	if (processor == PIPE_SHADER_VERTEX) {
+		assert(info); /* only CS may not have an info struct */
+
 		si_dump_descriptor_list(&sctx->vertex_buffers, shader_name[processor],
 					" - Vertex buffer", info->num_inputs, f);
 	}