Fix undefined in format-test

When `MoveCtor` performs `check_move_buffer`, the buffer allocator becomes null,
but then `MoveCtor` attempts to use it to allocate a dynamic buffer. This
succeeds nevertheless because a typical `std::allocator<char>::allocate` does
not use `this`, so it does not crash when `this` is null.

Fixes fmtlib#1344

Author: orivej

test/format-test.cc

@@ -271,7 +271,7 @@ static void check_move_buffer(
   EXPECT_EQ(alloc, buffer2.get_allocator().get());
 }
 
-TEST(MemoryBufferTest, MoveCtor) {
+TEST(MemoryBufferTest, MoveCtorInlineBuffer) {
   std::allocator<char> alloc;
   basic_memory_buffer<char, 5, TestAllocator> buffer((TestAllocator(&alloc)));
   const char test[] = "test";
@@ -281,6 +281,13 @@ TEST(MemoryBufferTest, MoveCtor) {
   // dynamic allocation.
   buffer.push_back('a');
   check_move_buffer("testa", buffer);
+}
+
+TEST(MemoryBufferTest, MoveCtorDynamicBuffer) {
+  std::allocator<char> alloc;
+  basic_memory_buffer<char, 5, TestAllocator> buffer((TestAllocator(&alloc)));
+  const char test[] = "testa";
+  buffer.append(test, test + 5);
   const char* inline_buffer_ptr = &buffer[0];
   // Adding one more character causes the content to move from the inline to
   // a dynamically allocated buffer.