fix: Add None case for finding max line number

	- Line number width is 1 if max line number is 0
	- Line number witdh is 0 if max line number is None
	- Update test_line_number_0 in tests/formatter.rs to reflect new behavior
	- Fix issue #57

Running tests/formatter.rs (target/debug/deps/formatter-d39ddd6093209828)

running 40 tests
...

test result: ok. 40 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

Author: chengr4

Diff for: src/renderer/display_list.rs

@@ -72,18 +72,17 @@ impl<'a> fmt::Debug for DisplayList<'a> {
 
 impl<'a> Display for DisplayList<'a> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let lineno_width = self.body.iter().fold(0, |max, set| {
+        let lineno_width = self.body.iter().fold(None, |max, set| {
             set.display_lines.iter().fold(max, |max, line| match line {
-                DisplayLine::Source { lineno, .. } => cmp::max(lineno.unwrap_or(0), max),
+                DisplayLine::Source { lineno, .. } => std::cmp::max(max, *lineno),
                 _ => max,
             })
         });
-        let lineno_width = if lineno_width == 0 {
-            lineno_width
-        } else if self.anonymized_line_numbers {
-            ANONYMIZED_LINE_NUM.len()
-        } else {
-            ((lineno_width as f64).log10().floor() as usize) + 1
+        let lineno_width = match lineno_width {
+            None => 0,
+            Some(_max) if self.anonymized_line_numbers => ANONYMIZED_LINE_NUM.len(),
+            Some(0) => 1,
+            Some(max) => (max as f64).log10().floor() as usize + 1,
         };
 
         let multiline_depth = self.body.iter().fold(0, |max, set| {