syslog: parse the port with atoi_short()

The hand rolled loop was here because atoi_short() reported "no digits"
but not overflow, so 65540 would have wrapped to 4 and opened a
connection to a port nobody asked for. #326 gave it the same bound this
loop was carrying, so the reason is gone.

Zero still covers all three ways to be wrong, except that "no digits"
and overflow now come back as a zero return and port zero as a zero
value, so both are tested.

Three xdata scratch variables go with the loop.

cmd_parser.c

@@ -65,11 +65,8 @@ __xdata	char save_cmd;
 
 __xdata uint8_t ip[4];
 
-/* Scratch for parse_syslog()'s port parser, in xdata: locals here would take
+/* Scratch for parse_syslog(), in xdata: a local here would take internal RAM
- * internal RAM the linker has none of. */
+ * the linker has none of. */
-__xdata uint16_t syslog_port_scratch;
-__xdata uint8_t  syslog_port_idx;
-__xdata uint8_t  syslog_port_digit;
 __xdata uint8_t  syslog_port_was_on;
 
 // These variables combined create a Fixed-capacity vector/bounded buffer.
@@ -1453,28 +1450,9 @@ void parse_syslog(void)
 			itoa_short(syslog_state.server_port); write_char('\n');
 			return;
 		}
-		/* Parsed here rather than with atoi_short(), which reports "no
+		/* atoi_short() returns zero for no digits and for a value past
-		 * digits" but not overflow - 65540 would quietly wrap to 4 and we
+		 * 65535; port zero is not a port either, so both tests are needed. */
-		 * would open a connection to a port nobody asked for. Checking
+		if (!atoi_short(cmd_words_b[2]) || !atoi_results_short) {
-		 * before each multiply keeps this in 16-bit arithmetic. */
-		syslog_port_scratch = 0;
-		syslog_port_idx = cmd_words_b[2];
-		for (;;) {
-			syslog_port_digit = cmd_buffer[syslog_port_idx];
-			if (syslog_port_digit < '0' || syslog_port_digit > '9')
-				break;
-			syslog_port_digit -= '0';
-			if (syslog_port_scratch > 6553
-			    || (syslog_port_scratch == 6553 && syslog_port_digit > 5)) {
-				syslog_port_scratch = 0;	/* too large - reject below */
-				break;
-			}
-			syslog_port_scratch = syslog_port_scratch * 10 + syslog_port_digit;
-			syslog_port_idx++;
-		}
-		/* Zero covers all three ways to be wrong: no digits at all, a value
-		 * past 65535, and an explicit "0", which is not a port either. */
-		if (!syslog_port_scratch) {
 			print_string("Invalid port\n");
 			return;
 		}
@@ -1485,7 +1463,7 @@ void parse_syslog(void)
 		if (syslog_port_was_on)
 			syslog_stop();
 		print_string("Setting new syslog port.\n");
-		syslog_state.server_port = syslog_port_scratch;
+		syslog_state.server_port = atoi_results_short;
 		if (syslog_port_was_on)
 			syslog_start();
 	}