fix corner case in collapse_vars (#4206)

fixes #4205

lib/compress.js

@@ -1494,14 +1494,22 @@ merge(Compressor.prototype, {
             }
 
             function handle_custom_scan_order(node, tt) {
+                if (!(node instanceof AST_BlockScope)) return;
+                // Skip (non-executed) functions
+                if (node instanceof AST_Scope) return node;
+                // Stop upon collision with block-scoped variables
+                if (node.variables && !node.variables.all(function(def) {
+                    return !lvalues.has(def.name);
+                })) {
+                    abort = true;
+                    return node;
+                }
                 // Scan object only in a for-in statement
                 if (node instanceof AST_ForIn) {
                     node.object = node.object.transform(tt);
                     abort = true;
                     return node;
                 }
-                // Skip (non-executed) functions
-                if (node instanceof AST_Scope) return node;
                 // Scan first case expression only in a switch statement
                 if (node instanceof AST_Switch) {
                     node.expression = node.expression.transform(tt);