Reject corrupt stream RDB with shared NACK across consumers

[sggeorgiev]

Summary

Detects and rejects corrupt stream RDB payloads where the same NACK (pending entry) is referenced by more than one consumer, which violates a stream data-structure.

Changes

• rdbLoadObject (stream consumer PEL loading): Added a guard that checks nack->consumer != NULL before assigning the consumer pointer. When a second consumer's PEL references a NACK that was already claimed by a prior consumer, the loader now reports a corrupt RDB error and aborts instead of silently overwriting the pointer. Without this check, two consumers share the same streamNACK, and freeing the first consumer's PEL leaves the second with a dangling pointer.
• corrupt-dump.tcl: Added a regression test that crafts a stream with two consumers (consumerA, consumerB) whose PELs both reference the same entry (1-0). The RESTORE command is expected to fail with "Bad data format", and the server must remain responsive (PING succeeds).

Benefits

• Fail-fast on corrupt data: The invariant violation is caught at load time with a clear diagnostic message rather than manifesting as a crash later during normal operation.
• Regression coverage: The crafted payload in the test ensures this class of corruption is permanently guarded against.

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Note

Medium Risk
Adds stricter validation during stream RDB/RESTORE loading; mistakes could cause previously-loadable (but corrupt) dumps to be rejected, and it touches stream consumer-group persistence logic that can affect load paths.

Overview
Prevents loading corrupt stream consumer-group state where multiple consumers reference the same global PEL streamNACK. During rdbLoadObject stream consumer PEL loading, it now fails fast if a resolved streamNACK already has a consumer set, instead of overwriting the pointer.

Adds an integration regression test that restores a crafted stream payload with a NACK shared by two consumers and asserts RESTORE fails with Bad data format while the server remains responsive.

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[sundb]

Pls check if we need to backport it to a version older than 8.6.
And it is necessary to confirm whether this corrupted test is applicable to version 8.6, because the rdb version of 8.8 has been increased, and possibly the version of this corrupted data is the latest one.