Fix potential stack buffer overflow in ecjpake
Fix potential stack buffer overflow in ecjpake

Two causes:
- the buffer is too short (missing 4 bytes for encoding id_len)
- the test was wrong

Would only happen when MBEDTLS_ECP_MAX_BITS == the bitsize of the curve
actually used (does not happen in the default config).

Could not be triggered remotely.

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