Use a std::unique_ptr for XRef

This makes memory handling a tiny bit more robust.
diff --git a/poppler/FontInfo.cc b/poppler/FontInfo.cc
index 3955789..7dc6276 100644
--- a/poppler/FontInfo.cc
+++ b/poppler/FontInfo.cc
@@ -69,27 +69,26 @@
     lastPage = doc->getNumPages() + 1;
   }
 
-  XRef *xrefA = doc->getXRef()->copy();
+  std::unique_ptr<XRef> xrefA(doc->getXRef()->copy());
   for (int pg = currentPage; pg < lastPage; ++pg) {
     page = doc->getPage(pg);
     if (!page) continue;
 
-    if ((resDict = page->getResourceDictCopy(xrefA))) {
-      scanFonts(xrefA, resDict, result);
+    if ((resDict = page->getResourceDictCopy(xrefA.get()))) {
+      scanFonts(xrefA.get(), resDict, &result);
       delete resDict;
     }
     annots = page->getAnnots();
     for (int i = 0; i < annots->getNumAnnots(); ++i) {
       Object obj1 = annots->getAnnot(i)->getAppearanceResDict();
       if (obj1.isDict()) {
-        scanFonts(xrefA, obj1.getDict(), result);
+        scanFonts(xrefA.get(), obj1.getDict(), &result);
       }
     }
   }
 
   currentPage = lastPage;
 
-  delete xrefA;
   return result;
 }