Fix namespace rule runtime crash

src/rules/namespace.js

@@ -131,6 +131,8 @@ exports.create = function namespaceRule(context) {
           break
         }
 
+        if (namespace.get(dereference.property.name) == null) return
+
         // stash and pop
         namepath.push(dereference.property.name)
         namespace = namespace.get(dereference.property.name).namespace

tests/files/commonjs-namespace/a.js

@@ -0,0 +1 @@
+export { default as b } from './b'

tests/files/commonjs-namespace/b.js

@@ -0,0 +1 @@
+module.exports = {}

tests/src/utils.js

@@ -84,4 +84,8 @@ export const SYNTAX_CASES = [
     code: 'export * from "./issue-370-commonjs-namespace/bar"',
     settings: { 'import/ignore': ['foo'] },
   }),
+
+  test({
+    code: 'import * as a from "./commonjs-namespace/a"; a.b',
+  }),
  ]