Properly initialize HtmlOutputDev::page to avoid SIGSEGV upon error exit.

Closes #742
diff --git a/utils/HtmlOutputDev.cc b/utils/HtmlOutputDev.cc
index 8d9728f..4bebe59 100644
--- a/utils/HtmlOutputDev.cc
+++ b/utils/HtmlOutputDev.cc
@@ -1089,6 +1089,7 @@
 {
   catalog = catalogA;
   fContentsFrame = nullptr;
+  page = nullptr;
   docTitle = new GooString(title);
   pages = nullptr;
   dumpJPEG=true;