In function:
static void add_music_decoder(const char *decoder)
{
        void *ptr = SDL_realloc(music_decoders, (num_decoders + 1) * sizeof (const char **));
        if (ptr == NULL) {
                return;  /* oh well, go on without it. */
        }
        music_decoders = (const char **) ptr;
        music_decoders[num_decoders++] = decoder;
}

Passing argument sizeof(char const **) to function SDL_realloc is suspicious. 

Logically it should be sizeof(char const *) instead of sizeof (char const **) 

In this particular case sizeof(char const **) happens to be equal to sizeof(char const *), but this is not a portable assumption. 
It reduces the understanding of the user.

So Patch should be,
void *ptr=SDL_realloc(music_decoders,(num_decoders+1) * sizeof(const char *));