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.
3da2e15e6b