fix closeDocument return #34

If we don't return 0 lua just exits on us, breaking return to file
chooser

djvu.c

@@ -117,6 +117,7 @@ static int closeDocument(lua_State *L) {
 
 		doc->context = NULL;
 	}
+	return 0;
 }
 
 static int getNumberOfPages(lua_State *L) {

pdf.c

@@ -79,6 +79,7 @@ static int closeDocument(lua_State *L) {
 		fz_free_context(doc->context);
 		doc->context = NULL;
 	}
+	return 0;
 }
 
 static int getNumberOfPages(lua_State *L) {