[fix](function) bitmap to base64 error length check

[Mryange]

Proposed changes

When calculating the length, mul ser_size twice.

Further comments

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[doris-robot]

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[Mryange]

run buildall

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[doris-robot]

TeamCity be ut coverage result: Function Coverage: 35.70% (9018/25263) Line Coverage: 27.34% (74553/272666) Region Coverage: 26.56% (38540/145114) Branch Coverage: 23.41% (19662/84000) Coverage Report: http://coverage.selectdb-in.cc/coverage/50a63ada3ce84d9809943b9015b15a2e81fdaf73_50a63ada3ce84d9809943b9015b15a2e81fdaf73/report/index.html

[github-actions[bot]]

PR approved by at least one committer and no changes requested.

[github-actions[bot]]

PR approved by anyone and no changes requested.