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>
diff --git a/cmd/optee.c b/cmd/optee.c
index d0d3729..e3aae5e 100644
--- a/cmd/optee.c
+++ b/cmd/optee.c
@@ -53,7 +53,7 @@
 {
 	int ret, value = 0;
 
-	if (strcmp(argv[1], NULL))
+	if (argc > 1)
 		value = hextoul(argv[1], NULL);
 
 	ret = hello_world_ta(value);