json_config: fix static analyzer false-positive

The variable can't be uninitialized, but gcc complains:

json_config.c:415:3: error: ‘params_len’ may be used uninitialized in
this function [-Werror=maybe-uninitialized]
   spdk_json_write_val_raw(w, cfg.params->start, params_len);

Change-Id: I845dcc19fa18c3583c420e66c16c11a3fdf99ad4
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/3626


Reviewed-by: Pawel Piatek <pawelx.piatek@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Ziye Yang <ziye.yang@intel.com>
Reviewed-by:  <dongx.yi@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>