apacheGH-40652: [C++] Enlarge dest buffer according to dest offset fo…

…r `CopyBitmap` benchmark (apache#40769)

### Rationale for this change

`CopyBitmap` benchmark doesn't make dest buffer large enough wrt. the possible dest offset, causing memory issues like when accessing the trailing bytes. 

### What changes are included in this PR?

Make the dest buffer wrt. dest offset.

### Are these changes tested?

The fix itself is test (benchmark).

### Are there any user-facing changes?

None.

* GitHub Issue: apache#40652

Authored-by: Ruoxi Sun <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

cpp/src/arrow/util/bit_util_benchmark.cc

@@ -449,7 +449,7 @@ static void CopyBitmap(benchmark::State& state) {  // NOLINT non-const reference
   const uint8_t* src = buffer->data();
   const int64_t length = bits_size - OffsetSrc;
 
-  auto copy = *AllocateEmptyBitmap(length);
+  auto copy = *AllocateEmptyBitmap(length + OffsetDest);
 
   for (auto _ : state) {
     internal::CopyBitmap(src, OffsetSrc, length, copy->mutable_data(), OffsetDest);