CLEANUP: buffer: use buffer_empty() instead of buffer_len()==0

A few places still made use of buffer_len()==0 to detect an empty
buffer. Use the cleaner and more efficient buffer_empty() instead.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 99f22f6..3e74ce5 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -1167,7 +1167,7 @@
 			buf->o -= ret;
 			done += ret;
 
-			if (likely(!buffer_len(buf)))
+			if (likely(buffer_empty(buf)))
 				/* optimize data alignment in the buffer */
 				buf->p = buf->data;