st/mesa: clamp gl_constants::MaxVarying against MAX_VARYING

Don't try to use more generic varying vars than core Mesa supports.
Fixes fd.o bug 29959.

src/mesa/state_tracker/st_extensions.c

@@ -168,6 +168,7 @@ void st_init_limits(struct st_context *st)
     * subtract the 2 colors to get the maximum number of varyings (generic
     * attributes) supported by a driver. */
    c->MaxVarying = screen->get_param(screen, PIPE_CAP_MAX_FS_INPUTS) - 2;
+   c->MaxVarying = MIN2(c->MaxVarying, MAX_VARYING);
 }