Make mbedtls_ssl_set_hostname safe to be called multiple times
Make mbedtls_ssl_set_hostname safe to be called multiple times

Zeroize and free previously set hostnames before overwriting
them. Also, allow clearance of hostname by providing NULL parameter.

ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls