Fix bad deallocate for zero-length allocations in jit allocator

[kg]

I think this is a bug.

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Pull Request Overview

This PR fixes a bug in the JIT allocator where zero-length allocations were causing incorrect deallocation behavior. The issue occurs when the allocator returns a reference to a static target for zero-length allocations, but then attempts to deallocate that reference as if it were a normal heap allocation.

• Adds a safety check to prevent deallocation of the zero-length allocation target
• Protects against potential crashes or undefined behavior during memory cleanup

[jakobbotsch]

CompAllocator::dellocate called below is a no-op, so no actual bug here. But the change seems good anyway to mirror what happens in alloc.