thread: add null check to spdk_io_channel_get_ctx

User of this function might expect that invalid channel ptr (NULL)
passed as an input would return invalid ctx ptr (NULL). If there is
no input check this function returns NULL + ctx size which is invalid
ptr but passing NULL check.

e.g.

ch = get_io_channel(); # returns NULL
ctx = channel_get_ctx(ch); # return non NULL
if (!ctx)
  err

ctx can be used and dereferenced later causing segfault.

Few existing SPDK code paths shall be considered (found by grep).

iscsi/iscsi_subsystem.c-1045-   ch = spdk_get_io_channel(&g_iscsi);
iscsi/iscsi_subsystem.c:1046:   pg = spdk_io_channel_get_ctx(ch);

nvmf/nvmf_rpc.c-2143-   ch = spdk_get_io_channel(ctx->tgt);
nvmf/nvmf_rpc.c:2144:   group = spdk_io_channel_get_ctx(ch);

blobfs/blobfs.c-544-    fs->md_target.md_io_channel = spdk_get_io_channel(&fs->md_target);
blobfs/blobfs.c:545:    fs->md_target.md_fs_channel = spdk_io_channel_get_ctx(fs->md_target.md_io_channel);

Signed-off-by: Jacek Kalwas <jacek.kalwas@intel.com>
Change-Id: I36cd41519f60188373837fd805242afb71934227
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/18279


Reviewed-by: Aleksey Marchuk <alexeymar@nvidia.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Community-CI: Mellanox Build Bot