fix: wrap CommandV allocations in try/finally to prevent memory leaks

[Bolt4243]

CommandV in MpvClient.cs allocated unmanaged memory via Marshal.AllocHGlobal but freed it outside a finally block. Any exception thrown mid-loop (e.g. from a subsequent Marshal.AllocHGlobal or Marshal.Copy call) would skip the free loop, leaking all previously allocated pointers as well as rootPtr.

This change wraps the allocation loop and the mpv_command call in a try block, with the free loop moved into the corresponding finally block. The per-pointer free also guards against IntPtr.Zero to safely skip slots that were never allocated.