Bug fix: tt_matrix should correctly take row vectors as input.

Without this fix, if you try to use tt_matrix with rows vector mode size it works, but will eventually crash.

Example:
>> row = [2, 3, 2];
>> tt = tt_zeros(row .* row);
>> tt_m = tt_matrix(tt, row, row); % No error here
>> tt_m * tt_m;
Error using  .* 
Matrix dimensions must agree.

Error in tt_matrix/mtimes (line 101)
    tt.ps = cumsum([1; tt.r(1:d).*n.*m.*tt.r(2:d+1)]);