Ensure application data records are not kept when fully processed

This commit fixes the following case: If a client is both expecting a
SERVER_HELLO and has an application data record that's partially
processed in flight (that's the situation the client gets into after
receiving a ServerHelloRequest followed by ApplicationData), a
subsequent call to mbedtls_ssl_read will set keep_current_message = 1
when seeing the unexpected application data, but not reset it to 0
after the application data has been processed. This commit fixes this.

It also documents and suggests how the problem might be solved in a
more structural way on the long run.

library/ssl_tls.c

@@ -6790,6 +6790,41 @@ int mbedtls_ssl_read( mbedtls_ssl_context *ssl, unsigned char *buf, size_t len )
         }
     }
 
+    /*
+     * TODO
+     *
+     * The logic should be streamlined here:
+     *
+     * Instead of
+     *
+     * - Manually checking whether ssl->in_offt is NULL
+     * - Fetching a new record if yes
+     * - Setting ssl->in_offt if one finds an application record
+     * - Resetting keep_current_message after handling the application data
+     *
+     * one should
+     *
+     * - Adapt read_record to set ssl->in_offt automatically
+     *   when a new application data record is processed.
+     * - Always call mbedtls_ssl_read_record here.
+     *
+     * This way, the logic of ssl_read would be much clearer:
+     *
+     * (1) Always call record layer and see what kind of record is on
+     *     and have it ready for consumption (in particular, in_offt
+     *     properly set for application data records).
+     * (2) If it's application data (either freshly fetched
+     *     or something already being partially processed),
+     *     serve the read request from it.
+     * (3) If it's something different from application data,
+     *     handle it accordingly, e.g. potentially start a
+     *     renegotiation.
+     *
+     * This will also remove the need to manually reset
+     * ssl->keep_current_message = 0 below.
+     *
+     */
+
     if( ssl->in_offt == NULL )
     {
         /* Start timer if not already running */
@@ -6996,6 +7031,7 @@ int mbedtls_ssl_read( mbedtls_ssl_context *ssl, unsigned char *buf, size_t len )
     {
         /* all bytes consumed */
         ssl->in_offt = NULL;
+        ssl->keep_current_message = 0;
     }
     else
     {