fix: Fix a linear space calculation bug in BipartiteBuf

This fixes a bug where the linear space until the end of the buffer was
not calculated correctly.
In debug builds, an assert would trigger however for release there could
be data corruption.

lockfree/spsc/bipartite_buf_impl.hpp

@@ -60,7 +60,7 @@ T *BipartiteBuf<T, size>::WriteAcquire(const size_t free_required) {
     const size_t r = _r.load(std::memory_order_acquire);
 
     const size_t free = CalcFree(w, r);
-    const size_t linear_space = size - r;
+    const size_t linear_space = size - w;
     const size_t linear_free = std::min(free, linear_space);
 
     /* Try to find enough linear space until the end of the buffer */