Use setter to set invocation name safely

Use setter to set invocation name safely

- Issue:
The invocation name previously set can be leaked.

- Solution:
The invocation name parameter is set by __vc_db_insert_commands_list()
without checking invocation itself. In most cases, invocation name is
not set by this function, but in special cases, memory leak can
occurred if use repeatedely set command list.
Thus, this patch prevents leak using setter function. The setter
function checks the invocation name paraemeter and safely changes it.

Change-Id: Ibe373fd51d5c2880ccea1d2b6699cd5614ffe49c
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>