bug#8215: possibly uninitialized variable lower_xoff in produce_glyphless_glyph

[Paul Eggert <eggert@cs.ucla.edu>]

I found this problem by compiling Emacs with GCC's -Wuninitialized flag.

The following code in the Emacs trunk src/xdisp.c's
produce_glyphless_glyph function might be using an uninitialized
variable:

       if (base_width >= width)
	{
	  /* Align the upper to the left, the lower to the right.  */
	  it->pixel_width = base_width;
	  lower_xoff = base_width - 2 - metrics_lower.width;
	}
       else
	{
	  /* Center the shorter one.  */
	  it->pixel_width = width;
	  if (metrics_upper.width >= metrics_lower.width)
	    lower_xoff = (width - metrics_lower.width) / 2;
	  else
	    upper_xoff = (width - metrics_upper.width) / 2;
	}
   ...
   if (it->glyph_row)
     append_glyphless_glyph (it, face_id, for_no_font, len,
			    upper_xoff, upper_yoff,
			    lower_xoff, lower_yoff);

The last call uses lower_xoff, but the last "else" does not initialize
lower_xoff.  The bug cannot occur if it->glyph_row is NULL, but I
don't see why that would necessarily be.  So I'm filing a bug report
so that someone who is more expert in this code can take a look at it.
In the meantime, I plan to work around the problem by initializing
lower_xoff to 0, with a FIXME explaining the situation: this shouldn't
introduce a bug, because at worst it will replace undefined behavior
with defined behavior.

I'm CC'ing this to Kenichi Handa, who committed the code in question.