encoding/xml: fix incorrect indirect code in chardata, comment, innerxml fields
The new tests in this CL have been checked against Go 1.7 as well
and all pass in Go 1.7, with the one exception noted in a comment
(an intentional change to omitempty already present before this CL).
CL 15684 made the intentional change to omitempty.
This CL fixes bugs introduced along the way.
Most of these are corner cases that are arguably not that important,
but they've always worked all the way back to Go 1, and someone
cared enough to file #19063. The most significant problem found
while adding tests is that in the case of a nil *string field with
`xml:",chardata"`, the existing code silently stops processing not just
that field but the entire remainder of the struct.
Even if #19063 were not worth fixing, this chardata bug would be.