Fix Bitset's core dump when copying from an empty Bitset

[songqing]

What do these changes do?

The Bitset can be constructed with size == 0, and when another Bitset copies from the empty Bitset, there will be core dump as the memcpy()

[sighingnow]

In both copy assignment and move assignment, I think you need to free/deallocate the existing data buffer (if not NULL) first before assigning?

[songqing]

In both copy assignment and move assignment, I think you need to free/deallocate the existing data buffer (if not NULL) first before assigning?
Yes, you are right. Done

[codecov-commenter]

Codecov Report

Merging #156 (d7e11ba) into master (515da59) will increase coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   63.94%   63.96%   +0.01%     
==========================================
  Files          67       67              
  Lines        6310     6310              
==========================================
+ Hits         4035     4036       +1     
+ Misses       2275     2274       -1     

Flag	Coverage Δ
app_tests	63.96% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
grape/utils/bitset.h	75.00% <ø> (ø)

... and 1 file with indirect coverage changes