tool_help: add checks to avoid unsigned wrap around

[x2018]

get_terminal_columns() can return cols from 1-10000 (21-10000 only via env COLUMNS), while the longdesc in option --help all is 47.
When cols < 47, the unsigned subtraction cols - longdesc wraps around to a large value, which is then passed as negative width to curl_mprintf after conversion to int.
Therefore, it is better to properly check the unsigned boundary to avoid unsigned wrap around and any other unexpected/undefined behavior.

[bagder]

It can be noted that curl's printf() implementation treats negative values just as it does zeros. I still agree with the fix.

[x2018]

It can be noted that curl's printf() implementation treats negative values just as it does zeros. I still agree with the fix.

Thanks! It would not crash or something dangerous but it will really affect the display width according to https://github.com/curl/curl/blob/master/lib/mprintf.c#L1015.

For example as below:

@test:~/curl-8.17.0/build/src# COLUMNS=47 ./curl --help all | head -n 3
     --abstract-unix-socket <path>  Connect via abstract Unix domain socket
     --alt-svc <filename>  Enable alt-svc with this cache file
     --anyauth  Pick any authentication method
@test:~/curl-8.17.0/build/src# COLUMNS=65 ./curl --help all | head -n 3
     --abstract-unix-socket <path>  Connect via abstract Unix domain socket
     --alt-svc <filename>  Enable alt-svc with this cache file
     --anyauth       Pick any authentication method
@test:~/curl-8.17.0/build/src# COLUMNS=29 ./curl --help all | head -n 3
     --abstract-unix-socket <path>  Connect via abstract Unix domain socket
     --alt-svc <filename>  Enable alt-svc with this cache file
     --anyauth       Pick any authentication method

When COLUMNS=29, the width will be the same as COLUMNS=65 because of the wrap around (47-18=29 and 47+18=65).