Bugfix
   * Ensure that calling mbedtls_rsa_free() twice is safe. This happens
     when some Mbed TLS library functions fail. Such a double-free was
     not safe when MBEDTLS_THREADING_C was enabled on platforms where
     freeing a mutex twice is not safe.
   * Fix a resource leak in a bad-arguments case of mbedtls_rsa_gen_key()
     when MBEDTLS_THREADING_C is enabled on platforms where initializing
     a mutex allocates resources.