Add explicit unsigned-to-signed integer conversion
Add explicit unsigned-to-signed integer conversion

The previous code triggered a compiler warning because of a comparison
of a signed and an unsigned integer.

The conversion is safe because `len` is representable by 16-bits,
hence smaller than the maximum integer.

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