vcl: fix drmemory warning in UniscribeLayout::DropGlyph()

It says there's access to uninitialized variable on that line,
so propbably the loop is running over.

Change-Id: I0226f82b20a6fbbc79af5bbb46af09830c3bc25a
(cherry picked from commit eba9a6c4)
Reviewed-on: https://gerrit.libreoffice.org/18107Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/win/source/gdi/winlayout.cxx

@@ -1495,7 +1495,7 @@ void UniscribeLayout::DropGlyph( int nStartx8 )
         DBG_ASSERT( nStart <= mnGlyphCount, "USPLayout::DropG overflow" );
 
         int j = pVI->mnMinGlyphPos;
-        while (mpOutGlyphs[j] == DROPPED_OUTGLYPH) j++;
+        while (j < mnGlyphCount && mpOutGlyphs[j] == DROPPED_OUTGLYPH) j++;
         if (j == nStart)
         {
             pVI->mnXOffset += ((mpJustifications)? mpJustifications[nStart] : mpGlyphAdvances[nStart]);