Drop initialized memory when parsing of emitted event origin fails

[sug0]

Introduced by #3268

When parsing an EventTrace<'a>, if a failure occurs, the memory that has been initialized is never dropped, due to the semantics of MaybeUninit values (which cannot be assumed to be initialized, therefore don't invoke Drop).

These event attributes are hidden behind the debug feature flag, so this bug isn't critical, as event origin attributes are only meant to be used for debugging purposes.