The undo_fixed_column struct is region-allocated via trail_stack, so its
destructor is never called. When m_fixed_val contains a big number (one
that doesn't fit in a small int), the heap-allocated memory for the mpq
numerator/denominator was never freed.
Fix by calling m_fixed_val.reset() in undo() to explicitly free the
heap memory before the region deallocates the struct.
Verified with macOS leaks tool:
- Before: 126 leaks (6048 bytes) on convert-jpg2gif-query-1584.smt2
- After: 0 leaks on normal completion, 10 leaks on timeout (unfinished trail)
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