Fixed the bug that a union type containing a struct with a flexible array member was accepted as a struct member or array element type.

test/err/bug2017-fam-element.c

@@ -1,9 +1,13 @@
 /* Bug #2017 - cc65 erroneously allows arrays of structs with flexible array members */
 
-struct z {
+typedef struct x {
     int a;
-    int c;
-    int b[];
-};
+    int b[];    /* Ok: Flexible array member can be last */
+} x;
 
-struct z y[3];          /* Should be an error */
+typedef union u {
+    int a;
+    x   x;      /* Ok: Union member can contain flexible array member */
+} u;
+
+union u y[3];  /* Should be an error */