cryptocop: fix assertion in create_output_descriptors()

size_t desc_len cannot be less than 0, test before the subtraction.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Mikael Starvik <starvik@axis.com>
Cc: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>