Volatile-ize all five places where we expect a PG_TRY block to restore

old memory context in plpython.  Before only one of them was marked
volatile, but per report from Zdenek Kotala, some compilers do the
wrong thing here.

src/pl/plpython/plpython.c

@@ -1,7 +1,7 @@
 /**********************************************************************
  * plpython.c - python as a procedural language for PostgreSQL
  *
- *	$PostgreSQL: pgsql/src/pl/plpython/plpython.c,v 1.137 2010/02/14 18:42:19 rhaas Exp $
+ *	$PostgreSQL: pgsql/src/pl/plpython/plpython.c,v 1.138 2010/02/18 23:50:06 tgl Exp $
  *
  *********************************************************************
  */
@@ -2763,7 +2763,7 @@ PLy_spi_prepare(PyObject *self, PyObject *args)
 	PyObject   *volatile optr = NULL;
 	char	   *query;
 	void	   *tmpplan;
-	MemoryContext oldcontext;
+	volatile MemoryContext oldcontext;
 
 	/* Can't execute more if we have an unhandled error */
 	if (PLy_error_in_progress)
@@ -2934,7 +2934,7 @@ PLy_spi_execute_plan(PyObject *ob, PyObject *list, long limit)
 	int			i,
 				rv;
 	PLyPlanObject *plan;
-	MemoryContext oldcontext;
+	volatile MemoryContext oldcontext;
 
 	if (list != NULL)
 	{
@@ -3081,7 +3081,7 @@ static PyObject *
 PLy_spi_execute_query(char *query, long limit)
 {
 	int			rv;
-	MemoryContext oldcontext;
+	volatile MemoryContext oldcontext;
 
 	oldcontext = CurrentMemoryContext;
 	PG_TRY();
@@ -3116,7 +3116,7 @@ static PyObject *
 PLy_spi_execute_fetch_result(SPITupleTable *tuptable, int rows, int status)
 {
 	PLyResultObject *result;
-	MemoryContext oldcontext;
+	volatile MemoryContext oldcontext;
 
 	result = (PLyResultObject *) PLy_result_new();
 	Py_DECREF(result->status);