incorrect sign when subtracting from sparse(0)

[David Bateman]

Thomas Kasper wrote:
> Bug report for Octave 3.0.0 configured for i686-pc-linux-gnu
> 
> Description:
> -----------
> 
>   subtraction of a sparse matrix from a sparse scalar with value 0
>   results in a wrong sign
> 
> Repeat-By:
> ---------
> 
>   octave:1> sparse (0) - sparse (1)
>   ans = Compressed Column Sparse (rows = 1, cols = 1, nnz = 1)
> 
>   (1, 1) ->  1
> 
>   % the same for a matrix-valued op2
>   octave:2> s = sprand (4,1,.5)
>   s =
> 
>   Compressed Column Sparse (rows = 4, cols = 1, nnz = 2)
> 
>     (2, 1) ->  0.96695
>     (3, 1) ->  0.26477
> 
>   octave:3> sparse (0) - s
>   ans =
> 
>   Compressed Column Sparse (rows = 4, cols = 1, nnz = 2)
> 
>     (2, 1) ->  0.96695
>     (3, 1) ->  0.26477
> 


I believe the attached patch should fix it, but have to do a complete
rebuild of Octave to check it..

D.
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