sftp.c: ensure minimum read packet size

For optimum performance we need to ensure we don't request tiny packets.

src/sftp.c

@@ -1367,13 +1367,18 @@ static ssize_t sftp_read(LIBSSH2_SFTP_HANDLE * handle, char *buffer,
 
         while(count > 0) {
             unsigned char *s;
-            uint32_t size = MIN(MAX_SFTP_READ_SIZE, count);
 
             /* 25 = packet_len(4) + packet_type(1) + request_id(4) +
                handle_len(4) + offset(8) + count(4) */
             uint32_t packet_len = (uint32_t)handle->handle_len + 25;
             uint32_t request_id;
 
+            uint32_t size = count;
+            if (size < buffer_size)
+                size = buffer_size;
+            if (size > MAX_SFTP_READ_SIZE)
+                size = MAX_SFTP_READ_SIZE;
+
             chunk = LIBSSH2_ALLOC(session, packet_len +
                                   sizeof(struct sftp_pipeline_chunk));
             if (!chunk)
@@ -1399,8 +1404,8 @@ static ssize_t sftp_read(LIBSSH2_SFTP_HANDLE * handle, char *buffer,
 
             /* add this new entry LAST in the list */
             _libssh2_list_add(&handle->packet_list, &chunk->node);
-            count -= size; /* deduct the size we used, as we might have
-                              to create more packets */
+            count -= MIN(size,count); /* deduct the size we used, as we might
+                                       * have to create more packets */
            _libssh2_debug(session, LIBSSH2_TRACE_SFTP,
                           "read request id %d sent (offset: %d, size: %d)",
                           request_id, (int)chunk->offset, (int)chunk->len);