IOFile: avoid clearing errors on null file struct

When performing a default compilation with recent GCC & glibc,
the use of -Werror=nonnull causes a build error.

The error is given as IOFile::ClearError() can call std::clearerr()
with a null file, which can trigger a null-pointer dereference in libc.

Change the std::clearerr() call to be conditional on a file being open.

Source/Core/Common/IOFile.h

@@ -116,6 +116,7 @@ public:
   void ClearError()
   {
     m_good = true;
+    if (IsOpen())
       std::clearerr(m_file);
   }