Initialize graphics in OutputDevice::getFallbackFont.

OutputDevice::getFallbackFont requires mpGraphics to be created/initialized.
Without the check and initialization OutputDevice::getFallbackFont will segfault.

Change-Id: I18225a24a6767933c8f1ff8f1cfd392543a95132
Reviewed-on: https://gerrit.libreoffice.org/14287Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/outdev/font.cxx

@@ -2027,6 +2027,11 @@ SalLayout* OutputDevice::getFallbackFont(ImplFontEntry &rFallbackFont,
     FontSelectPattern &rFontSelData, int nFallbackLevel,
     ImplLayoutArgs& rLayoutArgs) const
 {
+    // we need a graphics
+    if (!mpGraphics && !AcquireGraphics())
+        return nullptr;
+
+    assert(mpGraphics != nullptr);
     rFallbackFont.mnSetFontFlags = mpGraphics->SetFont( &rFontSelData, nFallbackLevel );
 
     rLayoutArgs.ResetPos();