Handle implicit close fence in MD031/blanks-around-fences to avoid null-dereference.

lib/md031.js

@@ -21,7 +21,11 @@ module.exports = {
       if ((includeListItems || !inItem) &&
           ((onTopFence && !isBlankLine(lines[i - 1])) ||
            (onBottomFence && !isBlankLine(lines[i + 1])))) {
-        const [ , prefix ] = line.match(codeFencePrefixRe);
+        const [ , prefix ] = line.match(codeFencePrefixRe) || [];
+        const fixInfo = (prefix === undefined) ? null : {
+          "lineNumber": i + (onTopFence ? 1 : 2),
+          "insertText": `${prefix}\n`
+        };
         addErrorContext(
           onError,
           i + 1,
@@ -29,10 +33,7 @@ module.exports = {
           null,
           null,
           null,
-          {
-            "lineNumber": i + (onTopFence ? 1 : 2),
-            "insertText": `${prefix}\n`
-          });
+          fixInfo);
       }
     });
   }

test/lists-with-fenced-code.md

@@ -57,3 +57,10 @@ Text
       fence {MD031:56} {MD031:58}
       ```
 1. Item
+
+Text
+
+1. Text
+   ```shell
+   fence {MD031:64} {MD031:65} {MD032:65} {MD040:66}
+  ```