Coverity points out that if the PDB file we're trying to read
has a header specifying a block_size of zero then we will
end up trying to divide by zero in pdb_ds_read_file().
Check for this and fail cleanly instead.

Fixes: Coverity CID 1458869
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Viktor Prutyanov <viktor.prutyanov@phystech.edu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Viktor Prutyanov <viktor.prutyanov@phystech.edu>
Message-id: 20210910170656.366592-3-philmd@redhat.com
Message-Id: <20210901143910.17112-3-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>