Fix reading metadata with zeros

[emasab]

Closes: #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

[confluent-cla-assistant]

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