Avoid double-extending past the end when showing source (#1377)

src/message.rs

@@ -57,7 +57,14 @@ impl Source {
     pub fn from_check(check: &Check, locator: &SourceCodeLocator) -> Self {
         let source = locator.slice_source_code_range(&Range {
             location: Location::new(check.location.row(), 0),
-            end_location: Location::new(check.end_location.row() + 1, 0),
+            // Checks can already extend one-past-the-end per Ropey's semantics. If they do, though,
+            // then they'll end at the start of a line. We need to avoid extending by yet another
+            // line past-the-end.
+            end_location: if check.end_location.column() == 0 {
+                check.end_location
+            } else {
+                Location::new(check.end_location.row() + 1, 0)
+            },
         });
         let num_chars_in_range = locator
             .slice_source_code_range(&Range {