fix corrupt completion cache detection

[ralphptorres]

The existing null byte check in update_aur_cache only detects corruption if a null byte appears in the first 1024 bytes of packages.aur. If the corrupt binary data happens to have no null in that window, the file passes validation and gets dumped raw to stdout, producing binary garbage in completions and a bash warning:

bash: warning: command substitution: ignored null byte in input

Replace the null-only check with a check for any non-printable byte (excluding newlines), since package names are plain ASCII. This reliably detects binary/corrupt cache files regardless of where null bytes fall.

Addresses the same symptom as #1553, but at the root cause rather than working around it in the shell completion script.