Fix missing error code in persist read error paths

[lonelybones]

In persist__chunk_client_msg_read_v56() and persist__chunk_base_msg_read_v56(), three error paths jump to the error cleanup label without setting rc, causing the functions to return success (0) on malformed input.

A crafted persistence file with a length underflow silently loads a corrupt message with NULL topic and payload pointers, leading to a NULL pointer dereference (SEGV) on the next persistence save.

Confirmed with AddressSanitizer: SEGV in strncmp called from persist__client_messages_save during broker shutdown.

Set rc to the appropriate error code before each goto.

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