Replace nullptr by NULL in the (hopefully) C++98 header file.
Replace nullptr by NULL in the (hopefully) C++98 header file.

ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe