crashfix-string deconstructor

validate we aren't either empty or using the reserved empty string before we release the pointed to value (as a note, the latter at least shouldn't have been required, but might as well be paranoid about it)

Engine/source/core/util/str.cpp

@@ -590,7 +590,8 @@ String::String(const UTF16 *str)
 
 String::~String()
 {
-   _string->release();
+   if (_string && _string != StringData::Empty())
+      _string->release();
 }
 
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