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

Fixes bug #178.
WM_SYMBOL long WildMidi_GetVersion (void) {
    return (LIBWILDMIDI_VERSION);
}

WM_SYMBOL long WildMidi_GetVersion (void) {
    return (LIBWILDMIDI_VERSION);
}
