It is not always possible to get the number of affected rows after a
partially executed statement, some drivers provide this information, but
some others, notable MS SQL native client one, do not.

Return -1 from get_affected_rows() to at least allow distinguishing this
situation from the one in which we can really know that no rows were
affected, in which case 0 would be returned.

Notice that this indirectly fixes the "Get affected rows" unit test
which was broken by the previous fix of #495 as it made the partial
insert actually work, while it was completely broken before because
"1\0" string couldn't be converted to a number and so the check for
get_affected_rows() wasn't being executed at all. But now that the
string is the correct "1", the check started being executed and failing.