IOFile: Change 'operator void*' into 'explicit operator bool'

'operator void*' is basically a pre-C++11-ism that was used, as C++03
only had the notion of implicit type-conversion operators, but not explicit type
conversion operators (allowing implicit conversion of a file handle to
bool can go downhill pretty quickly).

Source/Core/Common/FileUtil.h

@@ -211,7 +211,7 @@ public:
   bool IsOpen() const { return nullptr != m_file; }
   // m_good is set to false when a read, write or other function fails
   bool IsGood() const { return m_good; }
-  operator void*() { return m_good ? m_file : nullptr; }
+  explicit operator bool() const { return IsGood(); }
   std::FILE* ReleaseHandle();
 
   std::FILE* GetHandle() { return m_file; }