graphics/truevision: Fix build with Clang 6

matlist.cc:715:13: error: invalid operands to binary expression ('std::__1::ifstream'

(aka 'basic_ifstream<char>') and 'nullptr_t')
  if ( ifile != NULL ) {

http://beefy11.nyi.freebsd.org/data/head-i386-default/p478276_s338342/logs/errors/truevision-0.5.5.2_13.log