-LONGLONG_MIN is the undefined behavior in C.
longlong2decimal() used to do this:

  int longlong2decimal(longlong from, decimal_t *to) {
    if ((to->sign= from < 0))
      return ull2dec(-from, to);
    return ull2dec(from, to);

and later in ull2dec() (DIG_BASE is 1000000000):

  static int ull2dec(ulonglong from, decimal_t *to) {
    for (intg1=1; from >= DIG_BASE; intg1++, from/=DIG_BASE) {}

this breaks in gcc-5 at -O3. Here ull2dec is inlined into
longlong2decimal. And gcc-5 believes that 'from' in the
inlined ull2dec is always a positive integer (indeed, if it was
negative, then -from was used instead). So gcc-5 uses
*signed* comparison with DIG_BASE.

Fix: make a special case for LONGLONG_MIN, don't negate it