bug#23013: 25.0.92; assertion failure on marker outside text range in echo area

[Ken Raeburn]

No more than half an hour after I sat here wondering how many days
without a crash might suggest that maybe the change I reverted was
actually related to the crash, Emacs crashed; the same way as before, an
assertion failure on a marker position in " *Echo Area 0*", while
switching between buffers; the last stuff I typed was

  C-x b e @ M-backspace return

(I.e., switch buffer, start typing a partial name, notice the default is
what I want so delete what I typed, hit return.)

The w->pointm marker has charpos and bytepos set to 41 but the buffer
text field has gpt, z, gpt_byte, z_byte, all set to 1.

I've been running commit a1f221b, with d82f24b reverted, configured
with:
  '--prefix=/permabit/user/raeburn/dev/emacs/emacs-25/lx/Inst-5725167'
  '--with-x-toolkit=lucid' '--enable-checking' 'CPPFLAGS=-DMARKER_DEBUG'
  '--without-sound'

The marker debugging code hasn't flagged anything.

As I said earlier, the other two changes Eli suggested reverting don't
look like they should make any difference in my case. I'll keep looking
to see if there are some interesting looking places to insert
instrumentation to do range-checking on marker positions....