bug#26518: 26.0.50; M-x bubbles crashes emacs

[Stephen Berman]

0. emacs -Q
1. M-x bubbles
=> 
Thread 1 "emacs" received signal SIGSEGV, Segmentation fault.
arith_driver (code=code@entry=Amult, nargs=nargs@entry=2, 
    args=args@entry=0x7fffffffc8c0) at 
/home/steve/git/emacs-master/src/data.c:2807
2807          CHECK_NUMBER_OR_FLOAT_COERCE_MARKER (val);

Full backtrace attached.  The crash happens on exiting
bubbles--compute-offsets; stepping through that function with Edebung
shed no light for me.  Just before exiting that function I set a
breakpoint at arith_driver if val == 17 (the value on crashing according
to the backtrace), but that also failed to enlighten me.  However, when
I repeat the above recipe and between steps 0 and 1 visit bubble.el and
call M-x eval-buffer, then proceed to step 1, there is no crash and
bubbles starts normally.  So it looks like some variable is not getting
initialized, but I couldn't find which one.


In GNU Emacs 26.0.50 (build 6, x86_64-pc-linux-gnu, GTK+ Version 3.22.8)
 of 2017-04-15 built on rosalinde
Repository revision: 28718c4449972640c39a7be1b62ccc7f50c6e801
Windowing system distributor 'The X.Org Foundation', version 11.0.11901000

Configured using:
 'configure 'CFLAGS=-Og -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL GNUTLS LIBXML2 FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

bubbles_backtrace
Description: Binary data