crashtesting: bubble surface creation failure notification upwards

e.g. on converting moz858095-2.svg to odg

Change-Id: I1a222f1745cff9c4cb225f6c0c216072fb17b4f7
Reviewed-on: https://gerrit.libreoffice.org/58587Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/headless/svpgdi.cxx

@@ -1342,6 +1342,11 @@ std::shared_ptr<SalBitmap> SvpSalGraphics::getBitmap( long nX, long nY, long nWi
     }
 
     cairo_surface_t* target = SvpSalGraphics::createCairoSurface(pBitmap->GetBuffer());
+    if (!target)
+    {
+        SAL_WARN("vcl.gdi", "SvpSalGraphics::getBitmap, cannot create cairo surface");
+        return nullptr;
+    }
     cairo_t* cr = cairo_create(target);
 
     SalTwoRect aTR(nX, nY, nWidth, nHeight, 0, 0, nWidth, nHeight);