graphics/sswf: Fix build with Clang 6

compiler.c++:419:21: error: comparison between pointer and integer ('const char *' and 'int')

while(package_info != '\0') {
      ~~~~~~~~~~~~ ^  ~~~~

http://beefy11.nyi.freebsd.org/data/head-i386-default/p473790_s335878/logs/sswf-1.8.4_5.log