crypto/rsa/rsa_pss.c: silence coverity warning

Reported by Coverity Scan (CID 1439138)
[extended tests]
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7156)

crypto/rsa/rsa_pss.c

@@ -244,7 +244,7 @@ int RSA_padding_add_PKCS1_PSS_mgf1(RSA *rsa, unsigned char *EM,
 
  err:
     EVP_MD_CTX_free(ctx);
-    OPENSSL_clear_free(salt, sLen);
+    OPENSSL_clear_free(salt, (size_t)sLen); /* salt != NULL implies sLen > 0 */
 
     return ret;