fix invalid AST_For.init

Turns out the only place in `Compressor` which can generate invalid `AST_For.init` is within `drop_unused()`, so focus the fix-up efforts.

supercedes #1652
fixes #1656

lib/compress.js

@@ -1961,6 +1961,9 @@ merge(Compressor.prototype, {
                             return in_list ? MAP.splice(body) : make_node(AST_BlockStatement, node, {
                                 body: body
                             });
+                        } else if (is_empty(node.init)) {
+                            node.init = null;
+                            return node;
                         }
                     }
                     if (node instanceof AST_Scope && node !== self)
@@ -2327,7 +2330,6 @@ merge(Compressor.prototype, {
     };
 
     OPT(AST_For, function(self, compressor){
-        if (is_empty(self.init)) self.init = null;
         if (!compressor.option("loops")) return self;
         if (self.condition) {
             var cond = self.condition.evaluate(compressor);

test/compress/drop-unused.js

@@ -791,3 +791,17 @@ issue_1583: {
         }
     }
 }
+
+issue_1656: {
+    options = {
+        toplevel: true,
+        unused: true,
+    }
+    beautify = {
+        beautify: true,
+    }
+    input: {
+        for(var a=0;;);
+    }
+    expect_exact: "for (;;) ;"
+}