A bug in sparse matrix nonzeros counter

[manevalex]

Hello,
I believe that there is a bug in the num_nonzeros_ counter in the block_sparse_matrix.cc . There is a check for integer overflow in the constructor. This is OK. But this check is missing in the case where the counter is incremented e.g. in AppendRows() . As a result there are cases of num_nonzeros_ < 0 which results in undefined behavior. This problem is maybe also relevant in other types of matrices where nonzeros counter is used.

[sandwichmaker]

Thanks @manevalex I will take a look.