fix scan-build complaint

Some versions of scan-build complain about a possible NULL pointer passed to `strncmp`:

    src/ltr/ltrdigest_pdom_visitor.c:259:19: warning: Null pointer passed as 
    an argument to a 'nonnull' parameter
       if (!had_err && strncmp("Scores", buf, (size_t) 6) != 0) {
                       ^                 ~~~ 
    1 warning generated.
    make: *** [obj/ltrdigest_pdom_visitor.sb] Error 1

This commit adds an assertion to fix this.