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

Fixes bug #178.
WM_SYMBOL const char *WildMidi_GetString(uint16_t info) {
    static char WM_Version[] = "WildMidi Processing Library " PACKAGE_VERSION;
    switch (info) {
    case WM_GS_VERSION:
        return WM_Version;
    }
    return NULL;
}

WM_SYMBOL const char *WildMidi_GetString(uint16_t info) {
    static char WM_Version[] = "WildMidi Processing Library " PACKAGE_VERSION;
    switch (info) {
    case WM_GS_VERSION:
        return WM_Version;
    }
    return NULL;
}
