lib/mpi: check for possible zero length

Buggy client might pass zero nlimbs which is meaningless.
Added check for zero length.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Reviewed-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>

lib/mpi/mpiutil.c

@@ -58,6 +58,9 @@ mpi_ptr_t mpi_alloc_limb_space(unsigned nlimbs)
 {
 	size_t len = nlimbs * sizeof(mpi_limb_t);
 
+	if (!len)
+		return NULL;
+
 	return kmalloc(len, GFP_KERNEL);
 }