rune has a well-defined size, but C.int is implementation-specified.
Using one as the other should require an explicit conversion.

updates #13467

Change-Id: I53ab2478427dca790efdcc197f6b8d9fbfbd1847
Reviewed-on: https://go-review.googlesource.com/63730Reviewed-by: Ian Lance Taylor <iant@golang.org>