USB: f_accessory: Check for length before updating accessory string

It is possible that host sends SEND_STRING control request with zero
length and in that case, driver ends up setting string as NULL. This
results in CTS failure for accessory test. Hence add check for length
and update string only if length is nonzero.

Change-Id: I4594a46f830f45fc49f80a74ec6042383edf007d
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>