textproc/aspell: unbreak build with Clang 6 / GCC 7

modules/filter/tex.cpp:177:69: error: comparison between pointer and integer ('const char *' and 'int')

if (top.in_what == Parm || top.in_what == Opt || top.do_check == '\0')
                                                 ~~~~~~~~~~~~ ^  ~~~~

Reported by: antoine (via bug 224669)
Obtained from: upstream (Apsell 0.60.7)