Fix an error in how cksum parses checksum files

When cksum (or sha256sum, etc...) attempts to parse chekcusm files in
the "tagged output format" and it encounters the string "((", it
crashes due to an error in how it parses the checksum file. GNU
coreutils, by contrast, correctly recognizes this string as a syntax
error and ignores the line.

Author: frenchua

src/uucore/src/lib/features/checksum/validate.rs

@@ -281,6 +281,9 @@ impl LineFormat {
         // tagged format does not put a space before (filename)
 
         let par_idx = rest.iter().position(|&b| b == b'(')?;
+        if par_idx < 1 {
+            return None;
+        }
         let sub_case = if rest[par_idx - 1] == b' ' {
             SubCase::Posix
         } else {