message: added bounds check on unsigned level int
diff: @@ -84,7 +84,8 @@ impl<'a> Iterator for ArgumentSplitter<'a> {
  // Disable METHOD if enabled.
  b')' if self.flags & METHOD != 0 => self.flags ^= METHOD,
  // Otherwise decrement the parenthesis level.
  - b')' => level -= 1,
  + b')' if level > 0 =>
  + level -= 1,
  // Toggle double quote rules.
  b'"' => self.flags ^= DOUBLE,
  // Loop through characters until single quote rules are completed.
  