Fix iterators when advancing to negative numbers

in this case both n and diff are already negative, so we want to use +=,
not -=

src/delimited_index.h

@@ -71,11 +71,12 @@ class delimited_index : public index,
       return i_ == static_cast<const column_iterator*>(&it)->i_;
     }
     ptrdiff_t distance_to(const base_iterator& it) const override {
-      return (static_cast<ptrdiff_t>(
-                  static_cast<const column_iterator*>(&it)->i_) -
-              static_cast<ptrdiff_t>(i_)) /
-             ptrdiff_t(idx_->columns_);
+      ptrdiff_t i = i_;
+      ptrdiff_t j = static_cast<const column_iterator*>(&it)->i_;
+      ptrdiff_t columns = idx_->columns_;
+      return (j - i) / columns;
     }
+
     string value() const override {
       return idx_->get_trimmed_val(i_, is_first_, is_last_);
     }

src/index_collection.cc

@@ -64,13 +64,13 @@ void index_collection::full_iterator::advance(ptrdiff_t n) {
   }
   if (n < 0) {
     while (n < 0) {
-      auto diff = it_start_ - it_;
+      auto diff = -(it_ - it_start_);
       if (n > diff) {
-        it_ -= n;
+        it_ += n;
         return;
       }
-      it_ -= (diff + 1);
-      n += diff;
+      it_ += (diff + 1);
+      n -= diff;
       prev();
     }
     return;