[uniscribe] Fix buffer allocation

Email from Jonathan Kew:

My cygwin build kept aborting on certain test words when run with the
uniscribe backend. Turned out this was caused by a bug in the allocation
of scratch buffers in hb-uniscribe.cc.

Commit 2a17f9568d9724e045d2c1d660e007f3acd747d9 introduced a new line

  ALLOCATE_ARRAY (SCRIPT_VISATTR, vis_attr, glyphs_size);

but it failed to account for this in the computation of glyphs_size
(the number of glyphs for which scratch buffer space is available),
with the result that the vis_clusters array ends up overrunning the
end of the scratch buffer and clobbering the beginning of the buffer's
info[].

AFAICS, the vis_attr array is not actually used, so the simple fix is
to remove the line that allocates it. (If/when we -do- need to use
vis_attr for something, we'll need to add another term to the earlier
calculation of glyphs_size.)

With this patch, the uniscribe backend runs reliably again.

JK