cmd: optee: fix hello subcommand argument check

cmd: optee: fix hello subcommand argument check

When the `optee hello' subcommand is called, the do_optee_hello_world_ta()
function passes a NULL pointer to the strcmp() function while verifying its
input argument, which results in the following crash:

  => optee hello
  "Synchronous Abort" handler, esr 0x96000010, far 0x0

Fix this by verifying the number of input arguments instead.

Fixes: e3cf80fbe02d ("cmd: Add support for optee commands")
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Jens Wiklander <jens.wiklander@linaro.org>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Jerome Forissier <jerome.forissier@linaro.org>
Cc: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>