buffer: fix evbuffer_remove_buffer() with empty chain in front

buffer: fix evbuffer_remove_buffer() with empty chain in front

In case we have empty chain (chain that do not have any data, i.e. ->off
== 0) at the beginning of the buffer, and no more full chains to move to
the dst, we will skip moving of this empty chain, and hence
last_with_datap will not be adjusted, and things will be broken after.

Fix this by not relying on ->off, just count if we have something to
move that's it.

Test case from:
  https://github.com/envoyproxy/envoy/pull/6062

Fixes: #774