gallium/u_vbuf: drop min/max-scanning for empty indirect draws

gallium/u_vbuf: drop min/max-scanning for empty indirect draws

When building with asserts enabled, we'll end up triggering an assert
in pipe_buffer_map_range down this code-path, due to trying to map
an empty range. Even if we avoid that, we'll trigger another assert
a bit later, because u_vbuf_get_minmax_index returns a min-index of
-1 here, which gets promoted to an unsigned value, and gives us an
out-of-bounds buffer-mapping offset.

Since we can't really have a well-defined min/max range here when
the range is empty anyway, we should just drop this dance in the
first place. After all, no rendering is going to be produced.

This fixes a crash in dEQP-GLES31.functional.draw_indirect.random.0
on VirGL for me.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>