qseecom: use strnlen in qseecom_start_app

The appname received by qseecom_start_app() can be non-null terminated
string beyond the max size check. Change strlen to strnlen.

CRs-fixed: 2062061
Change-Id: I845d244336b5c81cedb2153d1303585f16819d58
Signed-off-by: Zhen Kong <zkong@codeaurora.org>