Buffer overflow in _grail_be_extract_gesture_attrs

[Christoph]

I observed segfaults occurring seemingly at random in libgeis.

I found that the function _grail_be_extract_gesture_attrs in libgeis/backend/grail/geis_grail_backend.c reads a cumulative transformation matrix from grail, which is a 3x3 matrix (9 floats). Immediately after this, it calls geis_frame_set_matrix passing this matrix, but geis_frame_set_matrix expects a 4x4 matrix and copies 16 floats.

This mismatch creates a buffer overflow which usually goes unnoticed, but sometimes raises SIGSEGV. The error shows up reliably every time when you compile libgeis with gcc's address sanitizer.

I attached a small patch that fixes the problem for me by converting the matrix to 4x4 before passing it to geis_frame_set_matrix. But I was unable to make 100% sure that I got the conversion right and that I did not accidentally transpose the matrix. I would be happy if someone could look over this patch.