Looks like it's time to update. They have found a bunch of security problems by themselves.
Even if we don't make use of ticket reuse or any of the other problematic modules, and the only thing that could affect our schannel implementation theoretically is the lack of sizeof(hostname) < 255 bounds checking.
Anyway, updating a library like this is always a good idea.
Used package: https://tls.mbed.org/download/mbedtls-2.1.2-apache.tgz with SHA-1 c99dfeaa27489f0e74e704e69a181f6ceb3db2a7.