Fix assert() condition in evbuffer_drain() for IOCP

In the case of iocp, in the for loop above, there is a situation where:
  remaining == chain->off == 0

And this happens due to CHAIN_PINNED_R() case (that is used only in
buffer_iocp.c)

Closes: libevent#630 (picked)

buffer.c

@@ -1146,7 +1146,7 @@ evbuffer_drain(struct evbuffer *buf, size_t len)
 		}
 
 		buf->first = chain;
-		EVUTIL_ASSERT(remaining < chain->off);
+		EVUTIL_ASSERT(remaining <= chain->off);
 		chain->misalign += remaining;
 		chain->off -= remaining;
 	}