Fix uninitialized variable in an edge case
Fix uninitialized variable in an edge case

If `context_buf = mbedtls_calloc( 1, buf_len )` failed,
`context_buf_len` was not initialized. Noticed by
`gcc -Os -Werror=maybe-uninitialized`.

This was only a problem in ssl_server2 (a test program), only with
MBEDTLS_SSL_CONTEXT_SERIALIZATION enabled.

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