Fix reading metadata with zeros

[emasab]

Closes: https://github.com/confluentinc/librdkafka/issues/4649. When retrieving offset metadata, if the binary value contained zeros and librdkafka was configured with strndup, part of the buffer after first zero contained uninitialized data instead of rest of metadata. Solved by avoiding to use strndup for copying metadata. Happens since: 0.9.0

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