fix corner case in collapse_vars

[alexlamsl]

fixes #5277

[alexlamsl]

@kzc in case you are keeping score, this week also sees Node.js v0.12 on GitHub Actions (ubuntu/Windows) often doing this:
https://github.com/mishoo/UglifyJS/runs/4745204864#step:4:363

Always the same three mocha tests which timed out when failiing, and while the other two might seem heavy, this one certainly shouldn't take more than 30s:

UglifyJS/test/mocha/cli.js

Lines 40 to 47 in b0df5d7

	it("Should be able to filter comments correctly with `--comment <RegExp>`", function(done) {
	var command = uglifyjscmd + ' test/input/comments/filter.js --comments /r/';
	exec(command, function(err, stdout) {
	if (err) throw err;
	assert.strictEqual(stdout, "/*@preserve*/\n// bar\n");
	done();
	});
	});

UglifyJS/test/input/comments/filter.js

Lines 1 to 3 in b0df5d7

	// foo
	/*@preserve*/
	// bar

[kzc]

That's pretty bad - two or three times slower is one thing, but --self is 30 times slower than my machine. Those machines are definitely over-provisioned.