Add the missing check of BN_bn2hex return value

CLA: trivial

Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17578)

(cherry picked from commit 4dd085c03a)

providers/implementations/encode_decode/encode_key2text.c

@@ -80,6 +80,9 @@ static int print_labeled_bignum(BIO *out, const char *label, const BIGNUM *bn)
     }
 
     hex_str = BN_bn2hex(bn);
+    if (hex_str == NULL)
+        return 0;
+
     p = hex_str;
     if (*p == '-') {
         ++p;