Fix segfault when broker has no OffsetFetch support

[xJakub]

Fixes #4789

Before the fix (using the consumer from examples/):

$ ./consumer localhost:9092 some_group topic
% Subscribed to 1 topic(s), waiting for rebalance and messages...
Segmentation fault (core dumped)

After the fix:

$ ./consumer localhost:9092 some_group topic
% Subscribed to 1 topic(s), waiting for rebalance and messages...
% Consumer error: Failed to fetch committed offsets for 0 partition(s) in group "some_group": Local: Required feature not supported by broker

[cla-assistant]

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