Fix SWA double-free in disagg decode with MTP speculation (#25805)

Co-authored-by: Ke Bao <ispobaoke@gmail.com>

python/sglang/srt/mem_cache/swa_memory_pool.py

@@ -561,6 +561,8 @@ class SWATokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
         self.full_to_swa_index_mapping[alloc_full_indices[-swa_tail_len:]] = (
             alloc_swa_indices
         )
+        if swa_tail_len < extend_num_tokens:
+            self.full_to_swa_index_mapping[alloc_full_indices[:-swa_tail_len]] = 0
         return alloc_full_indices
 
     def alloc_decode(