bug#19774: 25.0.50; Emacs crashes when C-u <N> on a Gnus folder (reproducible)

[Stefan Monnier]

>> #8  0x0117b418 in Fvector (nargs=-1, args=0x158bcd0 <dumped_data+32528>)
>> #9  0x0111d977 in Fthis_single_command_keys () at 
> It looks like this_command_key_count is -1.

Or this_single_command_key_start > this_command_key_count.

> How can that happen?

Good question.

> Should we protect ourselves against this?

I'd suggest you add `eassert's everywhere we mess with
this_single_command_key_start and this_command_key_count, so see who's
the culprit.


        Stefan