llava : fix memory management bug (#5491)

* Fix memory management in llava and server code

Fixes this error:

llama_new_context_with_model: graph splits (measure): 3
Available slots:
 -> Slot 0 - max context: 6000
{"timestamp":1707926446,"level":"INFO","function":"main","line":2623,"message":"model loaded"}
all slots are idle and system prompt is empty, clear the KV cache
slot 0 - loaded image
slot 0 is processing [task id: 0]
slot 0 : kv cache rm - [0, end)
slot 0 - encoding image [id: 1]
munmap_chunk(): invalid pointer
Aborted

* Make it cleaner by checking size in batch free wrapper