database/sql: prevent closes slices from assigning to free conn
In function connectionCleanerRunLocked append to closing slice affects db.freeConns and vise versa. Sometimes valid connections are closed and some invalid not.
Change-Id: I5282f15be3e549533b7d994b17b2060db3c0e7da
GitHub-Last-Rev:
b3eb3ab6f49c036519f777fc7189e9507010c166
GitHub-Pull-Request: golang/go#49429
Reviewed-on: https://go-review.googlesource.com/c/go/+/362214
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>