ls -w should reject invalid values

[sylvestre]

This option does the following:

       -w, --width=COLS
              set output width to COLS.  0 means no limit

$ ls -w8 > /dev/null; echo $?
0

$ ls -w08; echo $?                                                                                                                                                                                                   
ls: invalid line width: '08'
2

$ ./target/debug/coreutils ls -w08 > /dev/null; echo $?                                                                                                                                                              
0

Tested by GNU here:
https://github.com/coreutils/coreutils/blob/master/tests/ls/w-option.sh#L30=

[sylvestre]

Fixed by @Ganneff