Fix GKI buffer leak with discovery information service reading

Fix GKI buffer leak with discovery information service reading

If the discovery information service of the LE client has
the fields model number, serial number, fw version, etc,
the service would allocate PKI buffer and never do anything
with it, so it would leak.  It looks like it should have
been assigned the a callback string array, however fixing
that still doesn't fix the leak because the code that receives
the string array, bta_hh_le_dis_cback(), never uses it and
never frees it.

I believe the semantic is that the string arrays are kept around
as a cache in the srvc engine connection structure,
so it's the srvc engine dealloc of the callback structure that
needs to also free the string buffers if they have been allocated.
After fixing the string array allocation, add code to free the
string array entries if they are not null.

Also fixed an off by one error in DIS_SrUpdate() that would also lead
to a GKI buffer leak.

Improve two string termination cases to use a simple set of the
last entry in the char array instead of memsetting the whole array
when most of it will be filled by a following memcpy.

Change-Id: I7905cd771dbbe166e3c2b42e019bac9f5a312877
Signed-off-by: Mike J. Chen <mjchen@google.com>