bug#8215: possibly uninitialized variable lower_xoff in produce_glyphless_glyph

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: eggert@cs.ucla.edu,  8215@debbugs.gnu.org,  Kenichi Handa <handa@gnu.org>
> Date: Sun, 06 Jun 2021 11:00:04 +0200
> 
> Nice catch.  :-)  I tried reading that function for a couple of minutes
> to try to make sense how lower_xoff should have been initialised, but I
> had to admit defeat.

The reason we didn't have the fix earlier is that the problem is only
visible when the default face's font is variable-pitch, otherwise the
offending code is never executed.  So it was hard to understand what
that "workaround" initialization caused.

Let me know if I should add some comments there to make the code's
intent and purpose more clear.