evp: fix coverity 1451509: argument cannot be negative

Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14620)

crypto/evp/e_rc4_hmac_md5.c

@@ -46,8 +46,12 @@ static int rc4_hmac_md5_init_key(EVP_CIPHER_CTX *ctx,
                                  const unsigned char *iv, int enc)
 {
     EVP_RC4_HMAC_MD5 *key = data(ctx);
+    const int keylen = EVP_CIPHER_CTX_key_length(ctx);
 
-    RC4_set_key(&key->ks, EVP_CIPHER_CTX_key_length(ctx), inkey);
+    if (keylen <= 0)
+        return 0;
+
+    RC4_set_key(&key->ks, keylen, inkey);
 
     MD5_Init(&key->head);       /* handy when benchmarking */
     key->tail = key->head;