libssh2_sftp_seek64: flush packetlist and buffered data

When seeking to a new position, flush the packetlist and buffered data
to prevent already received or pending data to wrongly get used when
sftp-reading from the new offset within the file.

src/sftp.c

@@ -1859,8 +1859,18 @@ libssh2_sftp_fstat_ex(LIBSSH2_SFTP_HANDLE *hnd,
 LIBSSH2_API void
 libssh2_sftp_seek64(LIBSSH2_SFTP_HANDLE *handle, libssh2_uint64_t offset)
 {
-    if(handle)
+    if(handle) {
         handle->u.file.offset = handle->u.file.offset_sent = offset;
+        /* discard all pending requests and currently read data */
+        sftp_packetlist_flush(handle);
+
+        /* free the left received buffered data */
+        if (handle->u.file.data_left) {
+            LIBSSH2_FREE(handle->sftp->channel->session, handle->u.file.data);
+            handle->u.file.data_left = handle->u.file.data_len = 0;
+            handle->u.file.data = NULL;
+        }
+    }
 }
 
 /* libssh2_sftp_seek