Skins2: blind attempt to fix a ThemeRepository crash

Top crash on Skins2 module on Win32
(cherry picked from commit 9e76fdbc6e1e1243c59126688f23141329f10461)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/skins2/src/theme_repository.cpp

@@ -73,6 +73,7 @@ ThemeRepository::ThemeRepository( intf_thread_t *pIntf ): SkinObject( pIntf )
 
     // retrieve skins from skins directories and locate default skins
     map<string,string>::const_iterator itmap, itdefault;
+    bool b_default_found = false;
     for( itmap = m_skinsMap.begin(); itmap != m_skinsMap.end(); ++itmap )
     {
         string name = itmap->first;
@@ -83,7 +84,10 @@ ThemeRepository::ThemeRepository( intf_thread_t *pIntf ): SkinObject( pIntf )
                     &text );
 
         if( name == "Default" )
+        {
             itdefault = itmap;
+            b_default_found = true;
+        }
     }
 
     // retrieve last skins stored or skins requested by user
@@ -98,7 +102,7 @@ ThemeRepository::ThemeRepository( intf_thread_t *pIntf ): SkinObject( pIntf )
                          current.c_str(), b_readable ? "" : "NOT" );
 
     // set the default skins if given skins not accessible
-    if( !b_readable )
+    if( !b_readable && b_default_found )
         current = itdefault->second;
 
     // save this valid skins for reuse