bug#22012: 25.1.50; gdb broken by (json-string-format "doesn't start with `\"'!")

[martin rudalics]

> I suspect the undo changes, and the fact that they caused running Lisp
> and could trigger GC in the middle of fiddling with the buffer's gap.
> They were committed to master before the branch was cut.  That's why I
> asked about your update: that hole was plugged 2 days ago on the
> emacs-25 branch.

This also means that some bug reports we got over the past weeks are
unreliable.  And we might get more of them for master/trunk if we don't
merge the changes back soon.

martin