llama : suppress unary minus operator warning (#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
```
