Fixed problem in AssignExp where the result value might be uninitialized. see mini/assign1.d

tests/mini/assign1.d

@@ -0,0 +1,17 @@
+module mini.assign1;
+
+extern(C) int printf(char*, ...);
+
+struct X
+{
+    int a;
+    alias a b;
+}
+void main()
+{
+    X e = void;
+    e.a = e.b = 5;
+    printf("%d - %d\n", e.a, e.b);
+    assert(e.a == 5);
+    assert(e.a == e.b);
+}