Printf: Alternative hex fails with a 0 value

# Summary

Printf crashes when using alternative hex with a 0 value.

## Code
```
GNU:
> printf %#x 0
0

uutils: 
> cargo run printf %#x 0
printf: warning: '0': expected a numeric value
0x0error: process didn't exit successfully: `target\debug\coreutils.exe printf %#x 0` (exit code: 1)
```
