Avoid possible longjmp-induced logic error in PLy_trigger_build_args.

The "pltargs" variable wasn't marked volatile, which makes it unsafe
to change its value within the PG_TRY block.  It looks like the worst
outcome would be to fail to release a refcount on Py_None during an
(improbable) error exit, which would likely go unnoticed in the field.
Still, it's a bug.  A one-liner fix could be to mark pltargs volatile,
but on the whole it seems cleaner to arrange things so that we don't
change its value within PG_TRY.

Per report from Xing Guo.  This has been there for quite awhile,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/CACpMh+DLrk=fDv07MNpBT4J413fDAm+gmMXgi8cjPONE+jvzuw@mail.gmail.com

Author: tglsfdc

src/pl/plpython/plpy_exec.c

@@ -689,7 +689,7 @@ PLy_trigger_build_args(FunctionCallInfo fcinfo, PLyProcedure *proc, HeapTuple *r
 			   *pltrelid,
 			   *plttablename,
 			   *plttableschema,
-			   *pltargs = NULL,
+			   *pltargs,
 			   *pytnew,
 			   *pytold,
 			   *pltdata;
@@ -713,6 +713,11 @@ PLy_trigger_build_args(FunctionCallInfo fcinfo, PLyProcedure *proc, HeapTuple *r
 			return NULL;
 		}
 	}
+	else
+	{
+		Py_INCREF(Py_None);
+		pltargs = Py_None;
+	}
 
 	PG_TRY();
 	{
@@ -856,7 +861,7 @@ PLy_trigger_build_args(FunctionCallInfo fcinfo, PLyProcedure *proc, HeapTuple *r
 			PyObject   *pltarg;
 
 			/* pltargs should have been allocated before the PG_TRY block. */
-			Assert(pltargs);
+			Assert(pltargs && pltargs != Py_None);
 
 			for (i = 0; i < tdata->tg_trigger->tgnargs; i++)
 			{
@@ -870,8 +875,7 @@ PLy_trigger_build_args(FunctionCallInfo fcinfo, PLyProcedure *proc, HeapTuple *r
 		}
 		else
 		{
-			Py_INCREF(Py_None);
-			pltargs = Py_None;
+			Assert(pltargs == Py_None);
 		}
 		PyDict_SetItemString(pltdata, "args", pltargs);
 		Py_DECREF(pltargs);