Bytecode: fix logic in routine that substracts two sparse matrices

The logic of the dynSparseMatrix::Sparse_substract_SA_SB() routine was
incorrect.

In some cases, it would read past the last nonzero elements of the A matrix,
and consequently write past the number of allocated nonzero elements of the C
matrix.

This would lead to crashes and, probably, to wrong results under certain
circumstances.

Closes: #1652