gmp: Reject public keys with public exponent e < 3

This ensures that we don't load a key with e=1, which basically renders
RSA into a no-op.  Since keys are universally generated with e=65537 and
no reputable CA will sign keys with e=1, allowing this before didn't have
any real world impact.

src/libstrongswan/plugins/gmp/gmp_rsa_public_key.c

@@ -496,7 +496,7 @@ gmp_rsa_public_key_t *gmp_rsa_public_key_load(key_type_t type, va_list args)
 
 	this->k = (mpz_sizeinbase(this->n, 2) + 7) / BITS_PER_BYTE;
 
-	if (!mpz_sgn(this->e))
+	if (!mpz_sgn(this->e) || mpz_cmp_ui(this->e, 3) < 0)
 	{
 		destroy(this);
 		return NULL;