Fix SF bug #1669182. Handle string exceptions even if unraisable (ie in __del__).

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.5.1c1?
 Core and builtins
 -----------------
 
+- Bug #1669182: prevent crash when trying to print an unraisable error
+  from a string exception.
+
 - Bug #1653736: Properly discard third argument to slot_nb_inplace_power.
 
 - SF #151204:  enumerate() now raises an Overflow error at sys.maxint items.

Python/errors.c

@@ -590,7 +590,11 @@ PyErr_WriteUnraisable(PyObject *obj)
 		PyFile_WriteString("Exception ", f);
 		if (t) {
 			PyObject* moduleName;
-			char* className = PyExceptionClass_Name(t);
+			char* className = NULL;
+			if (PyExceptionClass_Check(t))
+				className = PyExceptionClass_Name(t);
+			else if (PyString_Check(t))
+				className = PyString_AS_STRING(t);
 
 			if (className != NULL) {
 				char *dot = strrchr(className, '.');