fixed FromUTF8() to accept NULL as well as len==npos; this fixes crashes when loading XML files in UTF-8 build with wxUSE_STL=1

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@46491 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/string.h

@@ -1207,11 +1207,19 @@ public:
 #if wxUSE_UNICODE_UTF8
     static wxString FromUTF8(const char *utf8)
     {
+      if ( !utf8 )
+          return wxEmptyString;
+
       wxASSERT( wxStringOperations::IsValidUtf8String(utf8) );
       return FromImpl(wxStringImpl(utf8));
     }
     static wxString FromUTF8(const char *utf8, size_t len)
     {
+      if ( !utf8 )
+          return wxEmptyString;
+      if ( len == npos )
+          return FromUTF8(utf8);
+
       wxASSERT( wxStringOperations::IsValidUtf8String(utf8, len) );
       return FromImpl(wxStringImpl(utf8, len));
     }