print/xmbibtex: Fix build with Clang 6

biblio.cc:2181:28: error: assigning to 'char *' from incompatible type 'char'

else {c[last-1]->doctype='\0';
                         ^~~~

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/xmbibtex-1.7_3.log