[C++] BitmapWriters clobber the first byte when length=0

[asfimport]

When a BitmapWriter or FirstTimeBitmapWriter is constructed with length=0 then Finish() is invoked, the byte at start_offset / 8 is zeroed

The following change causes the test to fail:

--- a/cpp/src/arrow/util/bit-util-test.cc
+++ b/cpp/src/arrow/util/bit-util-test.cc
@@ -274,6 +274,10 @@ TEST(FirstTimeBitmapWriter, NormalOperation) {
         auto writer = internal::FirstTimeBitmapWriter(bitmap, 10, 3);
         WriteVectorToWriter(writer, {0, 0, 0});
       }
+      {
+        auto writer = internal::FirstTimeBitmapWriter(bitmap, 13, 0);
+        WriteVectorToWriter(writer, {});
+      }
       {
         auto writer = internal::FirstTimeBitmapWriter(bitmap, 13, 3);
         WriteVectorToWriter(writer, {1, 0, 1});

Reporter: Ben Kietzman / @bkietz
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #3348

_{Note: This issue was originally created as ARROW-4186. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3348
#3348