Fix missing "volatile" in PLy_output().

Commit 5c3c3cd0a3046339 plastered "volatile" on a bunch of variables
in PLy_output(), but removed the one that actually mattered, ie the
one on "oldcontext".  This allows some versions of clang to generate
code in which "oldcontext" has been trashed when control reaches the
PG_CATCH block.  Per buildfarm member tick.

src/pl/plpython/plpy_plpymodule.c

@@ -404,7 +404,7 @@ PLy_output(volatile int level, PyObject *self, PyObject *args, PyObject *kw)
 	char *volatile datatype = NULL;
 	char *volatile table = NULL;
 	char *volatile schema = NULL;
-	MemoryContext oldcontext ;
+	volatile MemoryContext oldcontext;
 	PyObject *key, *value;
 	PyObject *volatile so;
 	Py_ssize_t pos = 0;