LU-14444 gss: handle empty reqmsg in sptlrpc_req_ctx_switch

LU-14444 gss: handle empty reqmsg in sptlrpc_req_ctx_switch

In sptlrpc_req_ctx_switch(), everything is already there to handle
the case of a ptlrpc_request that has an empty rq_reqmsg.
But assertions were left over at the beginning of the function, so
just remove them from here.

Lustre-change: https://review.whamcloud.com/41685
Lustre-commit: dfe87b089b662663ba125769866c98e803f89a8c

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I6ae1f8b9da9600d3b57b9efc9018c2461114f2fe
Reviewed-on: https://review.whamcloud.com/41858
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>