fix corner case in collapse_vars (#5870)

fixes #5869

lib/compress.js

@@ -3327,10 +3327,14 @@ Compressor.prototype.compress = function(node) {
             }
 
             function update_symbols(value, node) {
+                var clear_defined = node instanceof AST_SymbolRef && !node.defined;
                 var scope = node.scope || find_scope(scanner) || block_scope;
                 value.walk(new TreeWalker(function(node) {
                     if (node instanceof AST_BlockScope) return true;
-                    if (node instanceof AST_Symbol) node.scope = scope;
+                    if (node instanceof AST_Symbol) {
+                        if (clear_defined && node instanceof AST_SymbolRef) node.defined = false;
+                        node.scope = scope;
+                    }
                 }));
             }
 

test/compress/collapse_vars.js

@@ -10333,3 +10333,27 @@ issue_1666_undefined_strict: {
     }
     expect_stdout: true
 }
+
+issue_5869: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a, b, log = console.log;
+        log();
+        a.p = 0;
+        b = a;
+        log(b);
+    }
+    expect: {
+        var a, log = console.log;
+        log();
+        log(void (a.p = 0));
+    }
+    expect_stdout: TypeError("Cannot set properties of undefined")
+}