Fixed bug causing to reset dump_interval to 1 when input model -i is provided

[marco-rossi29]

Order of dump_interval modifications before bug fix:

1. dump_interval is initialized to 1 (see here)
2. dump_interval is updated according to -P input (see here)
3. If -i is present, dump_interval is updated according to dump_interval at time of model saving (see here)
4. if --preserve_performance_counter is not present (see here), dump_interval is WRONGLY reset to 1 (see here) rather than be reset to value after step 2