crypto_bignum_init_set() might fail in case of memory allocation
failures. These two cases within sswu() did not handle that properly,
i.e., a memory allocation failure could have resulted in dereferencing a
NULL pointer. Check the return value before proceeding to fix this.
Fixes: aeb022f8e51e ("SAE: Implement hash-to-element PT/PWE crypto routines")
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>