Reject AdditiveQuantizer with zero-bit code size on deserialization

[juancarpio27]

Summary:
Deserializing an AdditiveQuantizer-based index (residual quantizer,
local-search quantizer, and their product variants) validated that the
number of sub-quantizers M is positive, but not that the resulting
per-vector code size is nonzero. A crafted index can set every
sub-quantizer's bit width to 0, which makes the derived code size 0
while the index still declares a positive vector count.

Such an index ends up with an empty codes buffer whose data pointer is
null/unspecified. Decoding any vector then dereferences that pointer
inside the bitstring reader used to unpack per-vector codes, causing a
segfault on fully attacker-controlled input.

Reject this configuration right where the code size is derived, with a
descriptive exception. A quantizer that stores zero bits per vector
cannot decode anything and is never produced by real training, so this
only rejects malformed/crafted inputs.

Differential Revision: D115173394

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[meta-codesync]

This pull request has been merged in a9ecee2.