Fix memory leak for { $foo: bar }

[emanuele6]

{ BINDING: ExpD } wasn't freeing BINDING.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=60865

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OSS Fuzz insists we fix this asap.

[emanuele6]

I added a test that uses { BINDING } and { BINDING: foo }

[nicowilliams]

Thanks!