`xmlNodeGetContent()` returns `NULL` if `child` is `NULL`, which causes
a segmentation fault in `chunk_from_str()`.
Fixes: ec1eab0319 ("fixed XML parsing of TNCCS 1.1 ReasonString message")
`xmlNodeGetContent()` returns `NULL` if `child` is `NULL`, which causes
a segmentation fault in `chunk_from_str()`.
Fixes: ec1eab0319 ("fixed XML parsing of TNCCS 1.1 ReasonString message")