health: drop the constant-false branch the compiler warned about

The tcp slot dump printed the slot index behind `if (UIP_CONNS > 1)`, and
with one connection configured that is a constant-false condition around
live code: sdcc reported two "unreachable code" warnings for the body and
one "conditional flow changed by optimizer" for the loop that could only
run once.

Print the single slot directly and state the assumption as a build-time
check, so a build that raises UIP_CONNS fails loudly here instead of
quietly labelling every line as slot zero. Builds warning-free again.

cmd_parser.c

@@ -2162,13 +2162,14 @@ void health_show(void) __banked
 	print_byte(i);
 	write_char('\n');
 
-	/* The TCP connection slots (a single one on this configuration) */
-	for (i = 0; i < UIP_CONNS; i++) {
+	/* The TCP connection slot. One on this configuration; a build with more
+	 * has to say which slot each line is, so make that a visible decision. */
+#if UIP_CONNS != 1
+#error "health: the tcp dump prints a single slot, extend it for UIP_CONNS > 1"
+#endif
+	i = 0;
+	{
 		print_string("tcp ");
-		if (UIP_CONNS > 1) {
-			print_byte(i);
-			write_char(' ');
-		}
 		print_string("st ");
 		print_byte(uip_conns[i].tcpstateflags);
 		print_string(" tmr ");