solidgm: fix initialization warning

solidgm: fix initialization warning

Seeing the below warning while compiling nvme-cli:

ninja: Entering directory `.build'
[27/51] Compiling C object nvme.p/plugins_solidigm_solidigm-telemetry.c.o
../plugins/solidigm/solidigm-telemetry.c: In function ‘solidigm_get_telemetry_log’:
../plugins/solidigm/solidigm-telemetry.c:126:22: warning: ‘length’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   tl.configuration = json_tokener_parse_ex(jstok, conf_str, length);
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[51/51] Linking target nvme

Fix this by initializing the 'length' variable to zero.

Signed-off-by: Martin George <marting@netapp.com>