Code improvement. Also fix compiler warning.

[jjnicola]

What:
Use a char pointer instead of an zero-lenght array as kb_redis struct member.
Use g_strdup instead of strncpy for initializing kbr->path.
Free the path where necessary.

Why:

Declaring zero-length arrays in other contexts, including as interior members of structure objects or as non-member objects, is discouraged. Accessing elements of zero-length arrays declared in such contexts is undefined and may be diagnosed.

https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

How:

Checklist:

• Tests
• CHANGELOG Entry

[jjnicola]

Close #440