stream_xbar: Fix non-existing output assertion

[colluca]

PR #200 fixed an issue in the stream_xbar assertions, effectively enabling them when reset is not asserted:
https://github.com/pulp-platform/common_cells/pull/200/files#diff-872c7e67b01645ac7cc6cecf1dbb01b73f1b1882d8bb77089dcb27570c35ac23L169

Thanks to this, a bug in one of the assertions was revealed:

common_cells/src/stream_xbar.sv

Lines 172 to 176 in 52f83ee

	for (genvar i = 0; unsigned'(i) < NumInp; i++) begin : gen_sel_assertions
	assert property (@(posedge clk_i) disable iff (~rst_ni)
	(valid_i[i] |-> sel_i[i] < sel_oup_t'(NumOut))) else
	$fatal(1, "Non-existing output is selected!");
	end

sel_oup_t can represent values in the range [0, NumOut). Because of this, if for example NumOut is 32, the comparison will reduce to sel_i[i] < 0 which will never be satisfied.

This PR fixes this issue.

[niwis]

Thanks a lot!