fix corner case in collapse_vars (#5494)

fixes #5493

lib/compress.js

@@ -2831,6 +2831,7 @@ Compressor.prototype.compress = function(node) {
                     }
                     return find_stop_logical(parent, op, level);
                 }
+                if (parent instanceof AST_Await) return find_stop_value(parent, level + 1);
                 if (parent instanceof AST_Binary) {
                     var op;
                     if (parent.left === node || !lazy_op[op = parent.operator]) {

test/compress/awaits.js

@@ -2961,3 +2961,24 @@ issue_5478: {
     expect_stdout: "PASS"
     node_version: ">=8"
 }
+
+issue_5493: {
+    options = {
+        collapse_vars: true,
+        reduce_vars: true,
+    }
+    input: {
+        (async function(a) {
+            var b = await [ 42 || b, a = b ];
+            console.log(a);
+        })();
+    }
+    expect: {
+        (async function(a) {
+            var b = await [ 42 || b, a = b ];
+            console.log(a);
+        })();
+    }
+    expect_stdout: "undefined"
+    node_version: ">=8"
+}