Fix some subtle UB found by MSan. (#4093)

Technically, any small size buffer's lifetime has ended by the time we
get to the base's destructor. This means its no longer valid to access
table contents if stored there from the base destructor. We need to
handle destruction in the table class instead.

This ends up being a trivial change because the logic is already
factored out, we just need to call it from a different point.

common/raw_hashtable.h

@@ -442,7 +442,10 @@ class BaseImpl {
       : view_impl_(alloc_size, nullptr),
         growth_budget_(growth_budget),
         small_alloc_size_(small_alloc_size) {}
-  ~BaseImpl();
+
+  // Destruction must be handled by the table where it can destroy entries in
+  // any small buffer, so make the base destructor protected but defaulted here.
+  ~BaseImpl() = default;
 
   // NOLINTNEXTLINE(google-explicit-constructor): Designed to implicitly decay.
   operator ViewImplT() const { return view_impl(); }
@@ -575,6 +578,7 @@ class TableImpl : public InputBaseT {
   TableImpl(TableImpl&& arg) noexcept;
   auto operator=(const TableImpl& arg) -> TableImpl&;
   auto operator=(TableImpl&& arg) noexcept -> TableImpl&;
+  ~TableImpl();
 
   // Resets the hashtable to its initial state, clearing all entries and
   // releasing all memory. If the hashtable had an SSO buffer, that is restored
@@ -841,11 +845,6 @@ auto ViewImpl<InputKeyT, InputValueT, InputKeyContextT>::ComputeMetricsImpl(
   return metrics;
 }
 
-template <typename InputKeyT, typename InputValueT, typename InputKeyContextT>
-BaseImpl<InputKeyT, InputValueT, InputKeyContextT>::~BaseImpl() {
-  Destroy();
-}
-
 // TODO: Evaluate whether it is worth forcing this out-of-line given the
 // reasonable ABI boundary it forms and large volume of code necessary to
 // implement it.
@@ -1596,6 +1595,11 @@ auto TableImpl<InputBaseT, SmallSize>::operator=(TableImpl&& arg) noexcept
   return *this;
 }
 
+template <typename InputBaseT, ssize_t SmallSize>
+TableImpl<InputBaseT, SmallSize>::~TableImpl() {
+  this->Destroy();
+}
+
 // Reset a table to its original state, including releasing any allocated
 // memory.
 template <typename InputBaseT, ssize_t SmallSize>