NextObj fix (#71113)

this is to fix #70231.

for regions we could run into this situation -

object is the last object before heap_segment_allocated (hs)
T0 calls NextObj, gets next obj which starts at heap_segment_allocated (hs)
T1 changes ephemeral_heap_segment to hs
T0 does these comparisons
        (nextobj >= heap_segment_allocated(hs) && hs != hp->ephemeral_heap_segment) ||
        (nextobj >= hp->alloc_allocated))
both still false because alloc_allocated hasn't been changed just yet (and the old alloc_allocated is larger than nextobj)

T0 validates next obj, concludes its m_alignpad is not 0, asserts

T1 forms an allocation context starting at heap_segment_allocated, clears memory so by the time the dump is taken, m_alignpad is already cleared (actually we clear it in a_fit_segment_end)

I'm fixing this by saving the ephemeral_heap_segment and alloc_allocated and bail if nextobj is not on the saved eph seg or if those 2 saved values are no long in sync.

src/coreclr/gc/gc.cpp

@@ -44526,9 +44526,25 @@ Object * GCHeap::NextObj (Object * object)
         return NULL;
     }
 
-    if ((nextobj < heap_segment_mem(hs)) ||
-        (nextobj >= heap_segment_allocated(hs) && hs != hp->ephemeral_heap_segment) ||
-        (nextobj >= hp->alloc_allocated))
+    if (nextobj < heap_segment_mem (hs))
+    {
+        return NULL;
+    }
+
+    uint8_t* saved_alloc_allocated = hp->alloc_allocated;
+    heap_segment* saved_ephemeral_heap_segment = hp->ephemeral_heap_segment;
+
+    // We still want to verify nextobj that lands between heap_segment_allocated and alloc_allocated
+    // on the ephemeral segment. In regions these 2 could be changed by another thread so we need
+    // to make sure they are still in sync by the time we check. If they are not in sync, we just 
+    // bail which means we don't validate the next object during that small window and that's fine.
+    //
+    // We also miss validating nextobj if it's in the segment that just turned into the new ephemeral 
+    // segment since we saved which is also a very small window and again that's fine.
+    if ((nextobj >= heap_segment_allocated (hs)) && 
+        ((hs != saved_ephemeral_heap_segment) || 
+         !in_range_for_segment(saved_alloc_allocated, saved_ephemeral_heap_segment) ||
+         (nextobj >= saved_alloc_allocated)))
     {
         return NULL;
     }