llama : suppress unary minus operator warning (ggml-org#8448)

This commit updates the _try_copy lambda and moves the unary minus
operator to after the cast to int32_t.

The motivation for this that currently the following warning is
generated on windows:

```console
llama.cpp\src\llama.cpp(21147,30): warning C4146: unary minus operator
applied to unsigned type, result still unsigned
```

Author: danbev

src/llama.cpp

@@ -21144,7 +21144,7 @@ int32_t llama_token_to_piece(const struct llama_model * model, llama_token token
             size--;
         }
         if (length < (int32_t)size) {
-            return (int32_t) -size;
+            return -(int32_t) size;
         }
         memcpy(buf, token, size);
         return (int32_t) size;