pathencode: check result of .digest() method in sha1hash

Without this it was theoretically possible .digest() would fail and
return NULL but we wouldn't notice. Detected with cpychecker.

mercurial/pathencode.c

@@ -684,6 +684,8 b' static int sha1hash(char hash[20], const'
 
 	hashobj = PyObject_CallMethod(shaobj, "digest", "");
 	Py_DECREF(shaobj);
+	if (hashobj == NULL)
+		return -1;
 
 	if (!PyString_Check(hashobj) || PyString_GET_SIZE(hashobj) != 20) {
 		PyErr_SetString(PyExc_TypeError,