Fix possibly-lossy conversion warning from MSVC
Fix possibly-lossy conversion warning from MSVC

ssl_tls.c(4876): warning C4267: '=': conversion from 'size_t' to 'uint8_t', possible loss of data

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