local_serializer.cc: fix lossy implicit conversion compiler warning

[dconeybe]

Fix the following warning in local_serializer.cc:

Implicit conversion loses integer precision: 'std::vector<firebase::firestore::model::Segment>::size_type' (aka 'unsigned long') to 'pb_size_t' (aka 'unsigned int')

The fix is to explicitly cast to suppress the compiler warning. There should NEVER be a case where the size of the vector is greater than 2**32, so, in practice, this will never be a lossy conversion.

Fixes #9430

#no-changelog

[paulb777]

Thanks for fixing. Please add a changelog describing that this fixes the warnings introduced in 8.13.0.

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  Overall coverage changed from 86.31% (e5152af) to 86.31% (c938a91) by +0.00%.

  Filename	Base (e5152af)	Merge (c938a91)	Diff
  leveldb_key.cc	98.81%	98.38%	-0.43%
  leveldb_mutation_queue.cc	92.08%	93.58%	+1.51%
  local_serializer.cc	83.52%	83.57%	+0.05%

Test Logs

• Base (e5152af): https://github.com/firebase/firebase-ios-sdk/actions/runs/1954857365
• Merge (c938a91): https://github.com/firebase/firebase-ios-sdk/actions/runs/1955235269

1. https://storage.googleapis.com/firebase-sdk-metric-reports/1xj7JK4D0k.html

[dconeybe]

Thanks for fixing. Please add a changelog describing that this fixes the warnings introduced in 8.13.0.

Done.

[ncooke3]

LGTM. No warnings in the pod linting CI 👍