cmd: keep the history pointer inside the ring

The pointer is masked when it advances over the command text, then
incremented once more for the newline without masking, so it can come to
rest one past the end of the ring.

Both readers walk from the pointer with their own index masked every
step, so an index that sits outside the ring is never reached and the
walk does not end. `history` on the console spins there, and so does
`/cmd_log`, which the web page fetches whenever settings are saved: it
fills the transmit buffer and keeps going past it.

cmd_parser.c

@@ -1771,7 +1771,8 @@ void cmd_parser(void) __banked
 			// Copy last cmd-buffer to history.
 			cmd_history_ptr = (cmd_history_ptr + i) & CMD_HISTORY_MASK;
 			__xdata uint16_t p = cmd_history_ptr;
-			cmd_history[cmd_history_ptr++] = '\n';
+			cmd_history[cmd_history_ptr] = '\n';
+			cmd_history_ptr = (cmd_history_ptr + 1) & CMD_HISTORY_MASK;
 			do {
 				i--;
 				cmd_history[--p & CMD_HISTORY_MASK] = cmd_buffer[i];