Add changelog/test/comment for #696, seemingly fixed by 77a162b3

That commit changed rtlErrorCheck() to use astNewLINK()'s, instead of
directly doing multiple astAdd()'s, which results in there always being
only a single final astAdd(), even under -e/-ex/-exx.

This apparently fixed #696, and it pretty much makes sense too.
The expressions given to the Open or Draw String statements can have temp
vars such as those from iif(), and they will be destroyed at the end of the
statement, which means the next astAdd() after parsing the expression.

And then of course if rtlErrorCheck() is doing multiple astAdd()'s, maybe
even one before adding the statement using the currently registered temp
vars, that will cause trouble, because the temp vars will be destroyed too
early.