Subtle bug that causes stack corruption on Android (run ok on iOS)

evil.h

#include <openssl/md5.h>

string md5_evil(const char *message) {
    unsigned char digest[MD5_DIGEST_LENGTH];
    MD5((unsigned char*)message, strlen(message), digest);
    char md[2*MD5_DIGEST_LENGTH];
    for (int i = 0; i < MD5_DIGEST_LENGTH; i++) {
        sprintf(&md[i*2], "%02x", (unsigned int)digest[i]);
    }
    return string(md);
}

/* FIX */
string md5_angel(const char *message) {
    unsigned char digest[MD5_DIGEST_LENGTH];
    MD5((unsigned char*)message, strlen(message), digest);
    char md[2*MD5_DIGEST_LENGTH + 1];
    md[2*MD5_DIGEST_LENGTH] = '\0';
    for (int i = 0; i < MD5_DIGEST_LENGTH; i++) {
        sprintf(&md[i*2], "%02x", (unsigned int)digest[i]);
    }
    return string(md);
}