crypto: rsa - add a check for allocation failure

Static checkers insist that the mpi_alloc() allocation can fail so add
a check to prevent a NULL dereference.  Small allocations like this
can't actually fail in current kernels, but adding a check is very
simple and makes the static checkers happy.

Fixes: 6637e11e ("crypto: rsa - allow only odd e and restrict value in FIPS mode")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/rsa.c

@@ -220,6 +220,8 @@ static int rsa_check_exponent_fips(MPI e)
 	}
 
 	e_max = mpi_alloc(0);
+	if (!e_max)
+		return -ENOMEM;
 	mpi_set_bit(e_max, 256);
 
 	if (mpi_cmp(e, e_max) >= 0) {