Fix assertion with igncr when CR on input

[j123b567]

I have discovered a problem with function do_map. When I run picocom with

--imap igncr

it fails with assertion with CR character on input which it should just ignore.

do_map: Assertion 'n > 0 && n <= M_MAXMAP' failed.

After bisecting the code, it was introduced by commit 50b8e69

It seems that n is also valid for value 0, si this fix allow this value

[jacobq]

Seeing same problem still more than a year later (Running on RPi CM4):

picocom: picocom.c:817: do_map: Assertion `n > 0 && n <= M_MAXMAP' failed

# picocom --help
picocom v3.1

Compiled-in options:
  TTY_Q_SZ is 0
  HIGH_BAUD is enabled
  USE_FLOCK is enabled
  LINENOISE is enabled
  HISTFILE is: .picocom_history
  USE_CUSTOM_BAUD is enabled

# picocom --baud 9600 --databits 8 --parity n --stopbits 1 --flow n --imap ignlf  /dev/ttyUSB0
picocom v3.1

port is        : /dev/ttyUSB0
flowcontrol    : none
baudrate is    : 9600
parity is      : none
databits are   : 8
stopbits are   : 1
escape is      : C-a
local echo is  : no
noinit is      : no
noreset is     : no
hangup is      : no
nolock is      : no
send_cmd is    : sz -vv
receive_cmd is : rz -vv -E
imap is        : ignlf,
omap is        : 
emap is        : crcrlf,delbs,
logfile is     : none
initstring     : none
exit_after is  : not set
exit is        : no

Type [C-a] [C-h] to see available commands
Terminal ready
picocom: picocom.c:817: do_map: Assertion `n > 0 && n <= M_MAXMAP' failed.
                                                                          Aborted

[wsakernel]

Merged as wsakernel@db5f7c5. Thanks!