bug of shared log's size calculation

[shengofsun]

The shared log's size is calculated by "global_end_offset - global_start_offset". When all shared log are deleted, the "global_start_offset" will be reset to 0, while the "global_end_offset" will remain unchanged, in which case we will get a uncorrect "size" of shared log.

please see

rdsn/src/dist/replication/lib/mutation_log.cpp

Line 1668 in 6ce6d43

_global_start_offset =

for details.

Setting "_global_start_offset" to "_global_end_offset" may fix this, but we need to test it.

[qinzuoyan]

I think we can resolve this bug by simply check _log_files.size() > 0, as in #135