Fix stored-fields frame leak when PerField#finish throws

[michaeljmarshall]

Description

startStoredFields is called unconditionally before the try block in processDocument and processRowColumns. If fields[i].finish() throws (e.g. a Similarity whose computeNorm returns 0 for a non-empty field), finishStoredFields was skipped, leaving the codec writer with an unmatched startDocument. At flush, writer.finish(maxDoc) sees fewer completed docs than expected and throws "Wrote N docs, finish called with numDocs=M".

Fixed by wrapping the finish loop in a try/finally that guarantees finishStoredFields runs. Same fix applied to both processDocument and processRowColumns.

[martijnvg]

LGTM 👍

[javanna]

LGTM too