fix: extend gcc -Wnonnull hint into check_index

The earlier hint in impl_ref() only cut the warning chain at the
DistArray::begin/end call sites. find/set/owner/is_local/is_zero etc.
call check_index(i) and then dereference pimpl_ directly; gcc-14
doesn't propagate the non-null guarantee out of impl_ref() across
check_index's return boundary.

Add the same TILEDARRAY_UNREACHABLE hint at the bottom of each
check_index overload — check_index calls impl_ref() (which asserts
pimpl_) and check_local_index then delegates to check_index, so this
single point covers every public method that pre-validates via
check_index before bare pimpl_-> deref.

Author: evaleev

src/TiledArray/dist_array.h

@@ -1628,6 +1628,10 @@ class DistArray : public madness::archive::ParallelSerializableObject {
     TA_ASSERT(
         impl_ref().tiles_range().includes_ordinal(ord) &&
         "The ordinal index used to access an array tile is out of range.");
+    // After check_index returns, pimpl_ is non-null (impl_ref() asserts).
+    // Hint to gcc -Wnonnull so direct `pimpl_->...` uses in the caller
+    // don't trip on a propagated null `this` across inlining boundaries.
+    if (!pimpl_) TILEDARRAY_UNREACHABLE;
   }
 
   template <typename Index>
@@ -1636,6 +1640,7 @@ class DistArray : public madness::archive::ParallelSerializableObject {
     TA_ASSERT(
         impl_ref().tiles_range().includes(i) &&
         "The coordinate index used to access an array tile is out of range.");
+    if (!pimpl_) TILEDARRAY_UNREACHABLE;
   }
 
   template <typename Index1,