printf: renders \x as a literal but should error due to missing hexadecimal number

Steps to reproduce:
```
printf '\x'
```

What happens now: uutils `printf` renders this as a literal, as-is:
```
\x
```

What I expected to happen: GNU `printf` terminates with an error message:
```
printf: missing hexadecimal number in escape
```

Notes: this is causing a failure in the GNU test file `tests/printf/printf.sh`.