Fix uninitialized-variable compiler warning induced by commit e4128ee76.

I'm a little bit astonished that anyone's compiler would have failed to
complain about this.  The compiler surely does not know that is_procedure
means the function return value will be ignored.

src/pl/plpython/plpy_exec.c

@@ -210,6 +210,8 @@ PLy_exec_function(FunctionCallInfo fcinfo, PLyProcedure *proc)
 				ereport(ERROR,
 						(errcode(ERRCODE_DATATYPE_MISMATCH),
 						 errmsg("PL/Python procedure did not return None")));
+			fcinfo->isnull = false;
+			rv = (Datum) 0;
 		}
 		else if (proc->result.typoid == VOIDOID)
 		{