This CL resets the parent stack when printing a character or comment field struct.
In the case of XML elements, the previous parents stack must be considered. However,
charadata or comment fields can't be printed in other fields so it seems required to reset
the parent stack each time a chardata or comment field is printed.

Fixes #5072

Change-Id: I84f61c9bfce94133cd0c076c11211b9be5b4b1ac
Reviewed-on: https://go-review.googlesource.com/9910Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: roger peppe <rogpeppe@gmail.com>