vhdl: implement bounds checking for strand number

vhdl/splink.vhdl

@@ -146,10 +146,12 @@ begin
 						-- TODO udp_length, range check with MAX_STRAND_LEN
 						num_pixels <= MAX_STRAND_LEN;
 
-						-- FIXME bounds check
+						if unsigned(udp_data) >= NUM_STRANDS then
+							receive_state <= DROP;
+						else
 							active_strand <= to_integer(unsigned(udp_data));
-
 							receive_state <= FRAME_NUM;
+						end if;
 
 					when FRAME_NUM =>
 						if not some_strands_written then