Fix a bogus uninit var warning

Reviewed-by: NRichard Levitte <levitte@openssl.org>

ssl/tls13_enc.c

@@ -290,7 +290,7 @@ int tls13_change_cipher_state(SSL *s, int which)
     unsigned char *finsecret = NULL;
     EVP_CIPHER_CTX *ciph_ctx;
     const EVP_CIPHER *ciph = s->s3->tmp.new_sym_enc;
-    size_t ivlen, keylen, finsecretlen;
+    size_t ivlen, keylen, finsecretlen = 0;
     const unsigned char *label;
     size_t labellen;
     int ret = 0;