xpybConn_call should ensure ext is not NULL before using it

Found by Oracle's Parfait 1.2 static analyzer:
   Null pointer dereference (CWE 476): Read from null pointer ext
        at line 322 of /export/alanc/X.Org/amd64-parfait/xcb/xpyb/src/conn.c
            in function 'xpybConn_call'.
          Function xpybConn_load_ext may return constant 'NULL' at line 141,
              called at line 321.
          Null pointer introduced at line 141 in function 'xpybConn_load_ext'.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>

src/conn.c

@@ -319,7 +319,7 @@ xpybConn_call(xpybConn *self, PyObject *args, PyObject *kw)
 
     /* Check our dictionary of cached values */
     ext = xpybConn_load_ext(self, key);
-    if (!ext->present) {
+    if (ext != NULL && !ext->present) {
 	PyErr_SetString(xpybExcept_ext, "Extension not present on server.");
 	Py_DECREF(ext);
 	return NULL;