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

@@ -20886,6 +20886,20 @@ int32_t llama_token_to_piece(const struct llama_model * model, llama_token token
         return 0;
     }
 
+    // copy piece chars to output text buffer
+    // skip up to 'lstrip' leading spaces before copying
+    auto _try_copy = [=] (const char * token, size_t size) -> int32_t {
+        for (int32_t i = 0; i < lstrip && size && *token == ' '; ++i) {
+            token++;
+            size--;
+        }
+        if (length < (int32_t)size) {
+            return -(int32_t) size;
+        }
+        memcpy(buf, token, size);
+        return (int32_t) size;
+    };
+
     // if we have a cache - use it
     {
         const auto & cache = model->vocab.cache_token_to_piece;