Rather then write to the same variable per fakeConn, write to either
fakeConn or rowsCursor.

Fixes #20646

Change-Id: Ifc79f989bd1606b8e3ebecb1e7844cce3ad06e17
Reviewed-on: https://go-review.googlesource.com/45393Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>