Zero-initialize realm_error_t when converting from Status

[jbreams]

What, How & Why?

Fixes #6925.

☑️ ToDos

• 📝 Changelog update
• 🚦 Tests (or not relevant)
• C-API, if public C++ API changed
• bindgen/spec.yml, if public C++ API changed

[coveralls-official]

Pull Request Test Coverage Report for Build jonathan.reams_2938

• 11 of 11 (100.0%) changed or added relevant lines in 2 files are covered.
• 57 unchanged lines in 12 files lost coverage.
• Overall coverage increased (+0.01%) to 91.679%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/index_string.cpp	1	87.78%
src/realm/sync/noinst/client_impl_base.cpp	1	85.51%
src/realm/array_blobs_big.cpp	2	98.72%
src/realm/array_string.cpp	2	88.0%
test/fuzz_group.cpp	2	54.4%
src/realm/sync/network/network.cpp	3	89.66%
src/realm/util/file.cpp	3	82.2%
src/realm/util/serializer.cpp	3	90.03%
test/test_thread.cpp	3	66.67%
src/realm/bplustree.cpp	7	76.74%

Totals
Change from base Build 1856:	0.01%
Covered Lines:	231346
Relevant Lines:	252344

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💛 - Coveralls