firebase-ios-sdk

Fix a memory leak introduced in #6418 (#6778)

The root of the issue is that when serializing a singular filter, it is being treated as a unary filter before it is definitively established whether it is a unary filter or a field filter. The leak is caused by always serializing the unary filter's field path field for equality and non-equality filters -- if the filter's value turns out not to be NaN or null, the serialization code switches the filter's type to a field filter without clearing the partially-initialized unary filter. `pb_release` would not free the `unary_filter.field.field_path` member variable because it would consider the object not to be a unary filter.

Also a small refactoring to make the function easier to digest.