[llvm] [yaml2obj][MachO] Fix crash from integer underflow with invalid cmdsize (PR #165924)

[James Henderson via llvm-commits]

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@@ -31,3 +36,20 @@ LoadCommands:
     nsects:          0
     flags:           0
 ...
+
+## Test with an unknown load command value
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jh7370 wrote:

```suggestion
## Test with an unknown load command value.
```

https://github.com/llvm/llvm-project/pull/165924