Fix apply na overflow (#56000)

* fix groupby and apply UB

* fix/skip parser overflow

* revert unneeded

* feedback

Author: WillAyd

pandas/core/methods/selectn.py

@@ -139,7 +139,8 @@ def compute(self, method: str) -> Series:
 
         # arr passed into kth_smallest must be contiguous. We copy
         # here because kth_smallest will modify its input
-        kth_val = libalgos.kth_smallest(arr.copy(order="C"), n - 1)
+        # avoid OOB access with kth_smallest_c when n <= 0
+        kth_val = libalgos.kth_smallest(arr.copy(order="C"), max(n - 1, 0))
         (ns,) = np.nonzero(arr <= kth_val)
         inds = ns[arr[ns].argsort(kind="mergesort")]