GGML: Check for null buffers in get/set/copy tensor RPC endpoints

[struct]

The RPC operations RPC_CMD_SET_TENSOR, RPC_CMD_SET_TENSOR_HASH, RPC_CMD_GET_TENSOR, and RPC_CMD_COPY_TENSOR all require a backend buffer was previously allocated and located during the operation otherwise the code will subsequently crash with various NULL pointer dereferences. This adds checks for NULL pointer buffers after deserialize_tensor is called for each operation.

Tested the RPC changes loading various models over the RPC interface:

$ build/bin/rpc-server -d metal
...
create_backend: using Metal backend
Starting RPC server v2.0.0
  endpoint       : 127.0.0.1:50052
  local cache    : n/a
  backend memory : 49146 MB
Accepted client connection, free_mem=51533365248, total_mem=51539607552

llama-cli on the client side:

$ build/bin/llama-cli -m ~/Downloads/Meta-Llama-3.1-8B-Instruct-abliterated-Q6_K.gguf --rpc localhost:50052 -n 64 -ngl 99
...
> This is a test!
I'm ready to help. What's the purpose of this test?

and llama-bench

$ build/bin/llama-bench -rpc localhost:50052
warning: asserts enabled, performance may be affected
warning: debug build, performance may be affected
register_backend: registered backend Metal (1 devices)
register_device: registered device Metal (Apple M4 Max)
register_backend: registered backend BLAS (1 devices)
register_device: registered device BLAS (Accelerate)
register_backend: registered backend RPC (0 devices)
register_backend: registered backend CPU (1 devices)
register_device: registered device CPU (Apple M4 Max)
| model                          |       size |     params | backend    | threads |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | ------: | --------------: | -------------------: |
| llama 1B Q4_0                  | 606.54 MiB |     1.10 B | Metal,BLAS,RPC |      12 |           pp512 |     3115.99 ± 130.79 |
| llama 1B Q4_0                  | 606.54 MiB |     1.10 B | Metal,BLAS,RPC |      12 |           tg128 |         86.28 ± 2.04 |

build: e40e6f7a (5986)