[inplace_vector] Suppress GCC 11 bogus warning. NFCI.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98503
and duplicates seem to be related.

    inplace_vector.h:584:23: error: array subscript ‘sg14::inplace_vector<std::__cxx11::basic_string<char>, 5>[0]’
    is partly outside array bounds of ‘sg14::inplace_vector<std::__cxx11::basic_string<char>, 4> [1]’
    [-Werror=array-bounds]
      584 | set_size_(size_ - (oldend - newend));
          |           ^~~~~

include/sg14/inplace_vector.h

@@ -569,19 +569,19 @@ class inplace_vector : ipvbase_assignable<T>, ipvbase_t<T, N> {
     iterator erase(const_iterator first, const_iterator last) {
         auto ifirst = iterator(first);
         auto ilast = iterator(last);
-        if (ifirst != ilast) {
+        auto n = ilast - ifirst;
+        if (n != 0) {
             auto oldend = end();
 #if defined(__cpp_lib_trivially_relocatable)
             if constexpr (std::is_trivially_relocatable_v<value_type>) {
                 std::destroy(ifirst, ilast);
-                auto newend = std::uninitialized_relocate(ilast, oldend, ifirst);
-                set_size_(size_ - (oldend - newend));
+                std::uninitialized_relocate(ilast, oldend, ifirst);
+                set_size_(size_ - n);
                 return ifirst;
             }
 #endif // __cpp_lib_trivially_relocatable
-            auto newend = std::move(ilast, oldend, ifirst);
-            std::destroy(newend, oldend);
-            set_size_(size_ - (oldend - newend));
+            std::destroy(std::move(ilast, oldend, ifirst), oldend);
+            set_size_(size_ - n);
         }
         return ifirst;
     }