Avoid dereferencing NULL underlying configs from vconfig objects...

...in the storyscreen WML resolver procedure.

Fixes bug #13601.

changelog

@@ -16,6 +16,7 @@ Version 1.7.0+svn:
  * Miscellaneous and bugfixes:
    * Add strict compilation to cmake
    * Let cmake also use the CXXFLAGS and CFLAGS environment variables
+   * Fixed a segmentation fault with storyscreens using [if]
 
 Version 1.7.0:
  * AI:

players_changelog

@@ -14,6 +14,9 @@ Version 1.7.0+svn:
   * User interface
     * Improved the layout to take less space in certain cases.
 
+  * Miscellaneous and bugfixes
+    * Fixed a crash in some storyscreens.
+
 Version 1.7.0:
   * Campaigns
     * Delfador's Memoirs: new campaign added to mainline (Novice level,

src/storyscreen/part.cpp

@@ -164,6 +164,10 @@ part::TITLE_ALIGNMENT part::string_title_align(const std::string& s)
 
 void part::resolve_wml(const vconfig& cfg, game_state& gamestate)
 {
+	if(cfg.null()) {
+		return;
+	}
+
 	if(cfg.has_attribute("background")) {
 		background_file_ = cfg["background"];
 	}