ssl_tls: fix maximum output length

set maximum output length to MBEDTLS_SSL_OUT_CONTENT_LEN instead of
MBEDTLS_SSL_MAX_CONTENT_LEN.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 353c347..9dfa05b 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -151,7 +151,7 @@
 {
     int ret;
     size_t remaining, expansion;
-    size_t max_len = MBEDTLS_SSL_MAX_CONTENT_LEN;
+    size_t max_len = MBEDTLS_SSL_OUT_CONTENT_LEN;
 
 #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
     const size_t mfl = mbedtls_ssl_get_max_frag_len( ssl );