deshake: variable used uninitialized

Sometimes the scan finds nothing that qualifies for addition to
the array and pos is zero after the loops.  The code forces pos to
1 and the array is then processed as if it had one valid element in it,
producing some amusing but not very useful results.

I don't see the rationale for this.  If pos is zero coming out of the
loops, the only appropriate thing to do is set t->angle to zero.  The
attached patch does that.  It's worked properly in several tests so far.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>