Workaround apparent g++12 spurious error

In file included from ../testshared.cpp:12:
In constructor ‘pvxs::shared_array<E, Enable>::shared_array(size_t, V) [with V = std::nullptr_t; E = std::unique_ptr<unsigned int>; Enable = void]’,
    inlined from ‘void {anonymous}::testComplex()’ at ../testshared.cpp:225:57:
../../src/pvxs/sharedArray.h:288:17: error: pointer used after ‘void operator delete [](void*, std::size_t)’ [-Werror=use-after-free]
  288 |         :base_t(new _E_non_const[c], c)
      |                 ^~~~~~~~~~~~~~~~~~~
In member function ‘void pvxs::detail::sa_default_delete<E>::operator()(E*) const [with E = std::unique_ptr<unsigned int>]’,
    inlined from ‘std::__shared_count<_Lp>::__shared_count(_Ptr, _Deleter, _Alloc) [with _Ptr = std::unique_ptr<unsigned int>*; _Deleter = pvxs::detail::sa_default_delete<std::unique_ptr<unsigned int> >; _Alloc = std::allocator<void>; <template-parameter-2-4> = void; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’ at /usr/include/c++/12/bits/shared_ptr_base.h:958:11,
    inlined from ‘std::__shared_count<_Lp>::__shared_count(_Ptr, _Deleter) [with _Ptr = std::unique_ptr<unsigned int>*; _Deleter = pvxs::detail::sa_default_delete<std::unique_ptr<unsigned int> >; <template-parameter-2-3> = void; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’ at /usr/include/c++/12/bits/shared_ptr_base.h:939:57,
    inlined from ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(_Yp*, _Deleter) [with _Yp = std::unique_ptr<unsigned int>; _Deleter = pvxs::detail::sa_default_delete<std::unique_ptr<unsigned int> >; <template-parameter-2-3> = void; _Tp = std::unique_ptr<unsigned int>; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’ at /usr/include/c++/12/bits/shared_ptr_base.h:1478:17,
    inlined from ‘std::shared_ptr<_Tp>::shared_ptr(_Yp*, _Deleter) [with _Yp = std::unique_ptr<unsigned int>; _Deleter = pvxs::detail::sa_default_delete<std::unique_ptr<unsigned int> >; <template-parameter-2-3> = void; _Tp = std::unique_ptr<unsigned int>]’ at /usr/include/c++/12/bits/shared_ptr.h:232:48,
    inlined from ‘pvxs::detail::sa_base<E>::sa_base(A*, size_t) [with A = std::unique_ptr<unsigned int>; E = std::unique_ptr<unsigned int>]’ at ../../src/pvxs/sharedArray.h:136:10,
    inlined from ‘pvxs::shared_array<E, Enable>::shared_array(size_t, V) [with V = std::nullptr_t; E = std::unique_ptr<unsigned int>; Enable = void]’ at ../../src/pvxs/sharedArray.h:288:39,
    inlined from ‘void {anonymous}::testComplex()’ at ../testshared.cpp:225:57:
../../src/pvxs/sharedArray.h:92:35: note: call to ‘void operator delete [](void*, std::size_t)’ here
   92 |     void operator()(E* e) const { delete[] e; }
      |                                   ^~~~~~~~~~

test/testshared.cpp

@@ -218,14 +218,27 @@ void testFreezeThawVoid()
     testEq(C[0], 5u);
 }
 
+struct ImMobile {
+    int v = 0;
+    ImMobile() = default;
+
+    void store(int x) { v=x; }
+    int load() const { return v; }
+
+    ImMobile(const ImMobile&) = delete;
+    ImMobile(ImMobile&&) = delete;
+    ImMobile& operator=(const ImMobile&) = delete;
+    ImMobile& operator=(ImMobile&&) = delete;
+};
+
 void testComplex()
 {
     testDiag("%s", __func__);
 
-    shared_array<std::unique_ptr<uint32_t>> X(2, nullptr);
+    shared_array<ImMobile> X(2);
 
-    X[0] = decltype (X)::value_type{new uint32_t(4u)};
-    testEq(*X[0], 4u);
+    X[0].store(4);
+    testEq(X[0].load(), 4);
 }
 
 void testValue()