Fix 1-byte buffer overflow in mbedtls_mpi_write_string()
Fix 1-byte buffer overflow in mbedtls_mpi_write_string()

This can only occur for negative numbers. Fixes #2404.

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