fix corner case in unsafe evaluate

[alexlamsl]

fixes #4931

[alexlamsl]

@kzc TIL:

$ node -v
v8.17.0

$ echo 'for ({} of [ null ]);' | node
[stdin]:1
for ({} of [ null ]);
     ^

TypeError: Cannot destructure 'undefined' or 'null'.
    at [stdin]:1:6

$ echo 'for (var {} of [ null ]);' | node
[stdin]:1
for (var {} of [ null ]);
         ^

TypeError: Cannot destructure 'undefined' or 'null'.
    at [stdin]:1:1

$ node -v
v16.0.0

$ echo 'for ({} of [ null ]);' | node
[stdin]:1
for ({} of [ null ]);
     ^

TypeError: Cannot read property 'undefined' of null
    at [stdin]:1:6

$ echo 'for (var {} of [ null ]);' | node
[stdin]:1
for (var {} of [ null ]);
          ^

TypeError: Cannot destructure '.for' as it is null.
    at [stdin]:1:10

That last one seems to expose internal plumbing of v8 🤔

[kzc]

That last one seems to expose internal plumbing of v8 🤔

It does indeed.

For that matter, what is the 'undefined' property being referred to in TypeError: Cannot read property 'undefined' of null? Again, the node 8 error made more sense: TypeError: Cannot destructure 'undefined' or 'null'.. More exposed plumbing?

[alexlamsl]

Now try and make sense out of this one:
https://github.com/mishoo/UglifyJS/runs/2587286297?check_suite_focus=true#step:3:4119

$ node -v
v8.17.0

$ echo 'null[{} = 0]++' | node
[stdin]:1
null[{} = 0]++
        ^

TypeError: Cannot read property '0' of null
    at [stdin]:1:9

$ node -v
v16.0.0\x64

$ echo 'null[{} = 0]++' | node
[stdin]:1
null[{} = 0]++
          ^

TypeError: Cannot destructure '0' as it is null.
    at [stdin]:1:6

[kzc]

Good one.

Since destructuring is difficult to implement it ought to be difficult to explain too.