diagnostics: be sure to close the styling when lines are too short

bar.y:4.12-17: <error>error:</error> redefining user token number of foo
    -    4 | %token foo <error>123
    +    4 | %token foo <error>123</error>
           |            <error>^~~~~~</error>

* src/location.c (location_caret): Be sure to close.
* tests/diagnostics.at (Line is too short, and then you die): New.

src/location.c

@@ -461,6 +461,12 @@ location_caret (location loc, const char *style, FILE *out)
                   break;
                 }
             }
+          // The line is shorter than expected.
+          if (opened)
+            {
+              end_use_class (style, out);
+              opened = false;
+            }
           putc ('\n', out);
         }