Loading zero-padded negative number results in positive number

[guidovranken]

E.g. using mbedtls_mpi_read_string to load "-1" actually loads -1, but loading "-01" loads 1.

This appears to be new behavior. It was detected by OSS-Fuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32828

The revision range: e93095f...d520037

#include <mbedtls/bignum.h>

int main(void)
{
    mbedtls_mpi mpi;
    mbedtls_mpi_init(&mpi);
    if ( mbedtls_mpi_read_string(&mpi, 10, "-01") != 0 ) {
        goto error;
    }
    char out[1024];
    size_t outlen = sizeof(out);
    if ( mbedtls_mpi_write_string(&mpi, 10, out, outlen, &outlen) != 0 ) {
        goto error;
    }
    printf("%s\n", out);
error:
    return 0;
}