Silence clang static analyzer complaint about potential div by 0.

It can't, because if b is 0 the function would have
returned already; since b.m_hi is 0 b.m_lo can't be. The assert
reassures clang that this is the case.

libgnucash/engine/gnc-int128.cpp

@@ -770,6 +770,7 @@ GncInt128::div (const GncInt128& b, GncInt128& q, GncInt128& r) const noexcept
     q.m_hi = set_flags(hi, qflags);
     if (hi == 0 && bhi == 0) //let the hardware do it
     {
+        assert (b.m_lo != 0); // b.m_hi is 0 but b isn't or we didn't get here.
         q.m_lo = m_lo / b.m_lo;
         r.m_lo = m_lo % b.m_lo;
         return;