Fix no-sm2 with clang

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Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6298)

crypto/ec/ec_pmeth.c

@@ -124,7 +124,7 @@ static int pkey_ec_sign(EVP_PKEY_CTX *ctx, unsigned char *sig, size_t *siglen,
 
     if (ec_nid == NID_sm2) {
 #if defined(OPENSSL_NO_SM2)
-        ret = -1;
+        return -1;
 #else
         ret = SM2_sign(type, tbs, tbslen, sig, &sltmp, ec);
 #endif