reduce calls to SQLGetDiagRec

[pacman82]

Makes sense, even independently from #266. If it turns out it helps poor souls using erroneous drivers, which delete messages then fetching with an empty message buffer as a workaround, thats a plus.

[coveralls]

Pull Request Test Coverage Report for Build 2dd83737ebc62e6f965242d62e0b02dc8d2c5f1b-PR-280

• 8 of 8 (100.0%) changed or added relevant lines in 3 files are covered.
• 11 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 83.551%

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Files with Coverage Reduction	New Missed Lines	%
odbc-api/src/handles/diagnostics.rs	11	86.13%

Totals
Change from base Build ab3783668606ab56537df2c472f034672e6c62eb:	-0.2%
Covered Lines:	4160
Relevant Lines:	4979

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💛 - Coveralls