fix corner case in unused (#4134)

fixes #4133

lib/compress.js

@@ -2906,9 +2906,12 @@ merge(Compressor.prototype, {
             var fixed = this.fixed_value();
             if (!fixed) return true;
             this._dot_throw = return_true;
-            var result = fixed._dot_throw(compressor);
+            if (fixed._dot_throw(compressor)) {
                 delete this._dot_throw;
-            return result;
+                return true;
+            }
+            this._dot_throw = return_false;
+            return false;
         });
         def(AST_UnaryPrefix, function() {
             return this.operator == "void";

test/compress/drop-unused.js

@@ -2924,3 +2924,24 @@ forin_var_2: {
     }
     expect_stdout: "undefined"
 }
+
+issue_4133: {
+    options = {
+        evaluate: true,
+        merge_vars: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 1;
+        var b = [ a-- ], c = b && b[c];
+        console.log(a);
+    }
+    expect: {
+        var b = 1;
+        console.log(0);
+    }
+    expect_stdout: "0"
+}