[C++] used uninitialized warning

[asfimport]

    In file included from ../src/arrow/util/bit-util-test.cc:35:
    ../src/arrow/util/bit-stream-utils.h: In function 'void arrow::TestZigZag(int32_t)':
    ../src/arrow/util/bit-stream-utils.h:110:11: warning: 'buffer' is used uninitialized in this function [-Wuninitialized]
         memcpy(&buffered_values_, buffer_ + byte_offset_, num_bytes);
         ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #3013

_{Note: This issue was originally created as ARROW-3852. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3013
#3013