mesa: add an assertion in _mesa_enable_vertex_array_attrib()

Some of the enable/disable vertex array functions take a zero-based
generic index, while others take a VERT_ATTRIB_GENERIC0-based value.
Add an assertion to clarify that in one place.

Reviewed-by: Gert Wollny <gw.fossdev@gmail.com>

src/mesa/main/varray.c

@@ -1064,6 +1064,7 @@ _mesa_enable_vertex_array_attrib(struct gl_context *ctx,
                                  struct gl_vertex_array_object *vao,
                                  unsigned attrib)
 {
+   assert(attrib >= VERT_ATTRIB_GENERIC0);
    assert(attrib < ARRAY_SIZE(vao->VertexAttrib));
 
    if (!vao->VertexAttrib[attrib].Enabled) {