Add null check when querying for exports from a module

tests/cases/compiler/exportDeclarationInInternalModule.ts

@@ -0,0 +1,20 @@
+// @target: es5
+// @module: commonjs
+// @declaration: true
+
+class Bbb {
+}
+
+class Aaa extends Bbb { }
+
+module Aaa {
+    export class SomeType { }
+}
+
+module Bbb {
+    export class SomeType { }
+
+    export * from Aaa;      // this line causes the nullref
+}
+
+var a: Bbb.SomeType;

tests/cases/compiler/exportStarFromEmptyModule.ts

@@ -0,0 +1,25 @@
+// @target: es5
+// @module: commonjs
+// @declaration: true
+
+// @filename: exportStarFromEmptyModule_module1.ts
+export class A {
+    static r;
+}
+
+// @filename:exportStarFromEmptyModule_module2.ts
+// empty
+
+// @filename: exportStarFromEmptyModule_module3.ts
+export * from "exportStarFromEmptyModule_module2";
+export * from "exportStarFromEmptyModule_module1";
+
+export class A {
+    static q;
+}
+
+// @filename: exportStarFromEmptyModule_module4.ts
+import * as X from "exportStarFromEmptyModule_module3";
+var s: X.A;
+X.A.q;
+X.A.r; // Error