fix corner case in hoist_vars (#4894)

fixes #4893

lib/compress.js

@@ -1793,7 +1793,7 @@ merge(Compressor.prototype, {
                         return make_node(AST_Assign, candidate, {
                             operator: "=",
                             left: make_node(AST_SymbolRef, candidate.name, candidate.name),
-                            right: candidate.value
+                            right: candidate.value,
                         });
                     }
                     var assign = candidate;
@@ -8433,6 +8433,7 @@ merge(Compressor.prototype, {
             if (value) {
                 if (value instanceof AST_Sequence) value = value.clone();
                 var name = make_node(AST_SymbolRef, defn.name, defn.name);
+                name.fixed = value;
                 a.push(make_node(AST_Assign, defn, {
                     operator: "=",
                     left: name,

test/compress/hoist_vars.js

@@ -295,3 +295,81 @@ issue_4859: {
     }
     expect_stdout: "Infinity"
 }
+
+issue_4893_1: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        hoist_vars: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        function f() {
+            function g() {}
+            var a = null;
+            var b = null;
+            var c = null;
+            b.p += a = 42;
+            f;
+        }
+        try {
+            f();
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        try{
+            (function f() {
+                var b;
+                b = null;
+                b.p += 42;
+                f;
+            })();
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+}
+
+issue_4893_2: {
+    options = {
+        collapse_vars: true,
+        hoist_vars: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        function f() {
+            function g() {}
+            var a = null;
+            var b = null;
+            var c = null;
+            b.p += a = 42;
+            f;
+        }
+        try {
+            f();
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        try{
+            (function() {
+                var b;
+                b = null;
+                b.p += 42;
+            })();
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+}