http: make curl callbacks match contracts from curl header

Yes, these don't match perfectly with the void* first parameter of the
fread/fwrite in the standard library, but they do match the curl
expected method signature. This is needed when a refactor passes a
curl_write_callback around, which would otherwise give incorrect
parameter warnings.

Signed-off-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http-walker.c

@@ -185,7 +185,7 @@ static void process_alternates_response(void *callback_data)
 	struct active_request_slot *slot = alt_req->slot;
 	struct alt_base *tail = cdata->alt;
 	const char *base = alt_req->base;
-	static const char null_byte = '\0';
+	const char null_byte = '\0';
 	char *data;
 	int i = 0;
 
@@ -218,7 +218,7 @@ static void process_alternates_response(void *callback_data)
 		}
 	}
 
-	fwrite_buffer(&null_byte, 1, 1, alt_req->buffer);
+	fwrite_buffer((char *)&null_byte, 1, 1, alt_req->buffer);
 	alt_req->buffer->len--;
 	data = alt_req->buffer->buf;