SF patch #1630975: Fix crash when replacing sys.stdout in sitecustomize

When running the interpreter in an environment that would cause it to set
stdout/stderr/stdin's encoding, having a sitecustomize that would replace
them with something other than PyFile objects would crash the interpreter.
Fix it by simply ignoring the encoding-setting for non-files.

This could do with a test, but I can think of no maintainable and portable
way to test this bug, short of adding a sitecustomize.py to the buildsystem
and have it always run with it (hmmm....)
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index 90c9687..1880187 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -354,6 +354,8 @@
 {
 	PyFileObject *file = (PyFileObject*)f;
 	PyObject *str = PyString_FromString(enc);
+
+	assert(PyFile_Check(f));
 	if (!str)
 		return 0;
 	Py_DECREF(file->f_encoding);