graph: guard iswa kq_mask on its own buffer (#24294)
A SWA-only draft head (e.g. StepFun MTP) leaves the base sub-cache
empty, so its kq_mask buffer stays null and asserts at load. Guard
each mask on its own buffer in set_input and can_reuse, base and swa.
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>
