cmd: "port <n> name" without a name clears it instead of reading stale words

The name branch read the fourth word without checking that the line had
one, and the tokenizer does not reset unused word slots, so "port 5 name"
took the name from wherever the previous command's fourth word had been:
"port 5 name hello" followed by "port 5 name" gave "hello" again, and
after a longer line it gave the tail of that line. The web UI never sends
the form, the console can. With no argument the name is now empty, which
also gives the console a way to remove a name.

cmd_parser.c

@@ -901,7 +901,7 @@ void parse_port(void)
 		port_media_show(phy_settings.port);
 	} else if (cmd_compare(2, "name")) {
 		uint8_t i = 0;
-		while ( (i < PORT_NAME_SIZE-1) && (cmd_buffer[cmd_words_b[3] + i] != NUL) ) {
+		while ( cmd_words_len > 3 && (i < PORT_NAME_SIZE-1) && (cmd_buffer[cmd_words_b[3] + i] != NUL) ) {
 			port_names[phy_settings.port][i] = name_char(cmd_buffer[cmd_words_b[3] + i]);
 			i++;
 		}