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

Fixes bug #178.
static inline int wm_tolower(int c) {
    return ((wm_isupper(c)) ? (c | ('a' - 'A')) : c);
}

static inline int wm_tolower(int c) {
    return ((wm_isupper(c)) ? (c | ('a' - 'A')) : c);
}
