icu related crash

[shi-yan]

this line cause seg fault on my machine,

   const Normalizer2 *nfkc = Normalizer2::getNFKCInstance(errorCode);

https://github.com/kermitt2/pdfalto/blame/9553aba9a3df882d53bb82a2ecdcd9eeb25c7a0d/src/XmlAltoOutputDev.cc#L723C7-L723C7

both the error code and the returned pointer are not handled. in my case, I got U_FILE_ACCESS_ERROR and the pointer is null.