tighten an assertion check (based on a review comment, see trac #116)

git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@1907 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/dns/rdata/generic/nsec_47.cc

@@ -182,7 +182,7 @@ NSEC::toText() const {
         assert(i + 2 <= impl_->typebits_.size());
         const int block = impl_->typebits_.at(i);
         len = impl_->typebits_.at(i + 1);
-        assert(len >= 0 && len <= 32);
+        assert(len > 0 && len <= 32);
         i += 2;
         for (int j = 0; j < len; j++) {
             if (impl_->typebits_.at(i + j) == 0) {