Bugfix
   * Fix some cases in the bignum module where the library constructed an
     unintended representation of the value 0 which was not processed
     correctly by some bignum operations. This could happen when
     mbedtls_mpi_read_string() was called on "-0", or when
     mbedtls_mpi_mul_mpi() and mbedtls_mpi_mul_int() was called with one of
     the arguments being negative and the other being 0. Fixes #4643.