wildmidi_lib.c (WildMidi_Open, WildMidi_OpenBuffer): refuse to proceed if less then 18 bytes of input

Fixes bug #178.
static int WM_LoadConfig(const char *config_file) {
    return load_config(config_file, NULL);
}

static int WM_LoadConfig(const char *config_file) {
    return load_config(config_file, NULL);
}
