Add ubsan suppressions for integer overflows in the getchaintxstats RPC.

getchainstatstx line "int nTxDiff = pindex->nChainTx - past_block.nChainTx" can
trigger ubsan integer overflows when assumeutxo snapshots are loaded, from
subtracting unsigned values and assigning the result to a signed int.

The overflow behavior probably exists in current code but is hard to trigger
because it would require calling getchainstatstx at the right time with
specific parameters as background blocks are being downloaded. But the overflow
behavior becomes easier to trigger in the upcoming commit removing fake
nChainTx values, so a suppression needs to be added before then for CI to pass.

getchainstatstx should probably be improved separately in another PR to not
need this suppression, and handle edge cases and missing nChainTx values more
carefully.

…nected block

Use Assume macro as suggested bitcoin#29370 (comment)

The checks are changing slightly in the next commit, so try to explains the
ones that exist to avoid confusion
(bitcoin#29370 (comment))

The `PopulateAndValidateSnapshot` function introduced in
f6e2da5 from bitcoin#19806 has been setting fake
`nTx` and `nChainTx` values that can show up in RPC results (see bitcoin#29328) and
make `CBlockIndex` state hard to reason about, because it is difficult to know
whether the values are real or fake.

Revert to previous behavior of setting `nTx` and `nChainTx` to 0 when the
values are unknown, instead of faking them.

This commit fixes at least two assert failures in the (pindex->nChainTx ==
pindex->nTx + prev_chain_tx) check that would happen previously. Tests for
these failures are added separately in the next two commits.

Compatibility note: This change could result in -checkblockindex failures if a
snapshot was loaded by a previous version of Bitcoin Core and not fully
validated, because fake nTx values will have been saved to the block index. It
would be pretty easy to avoid these failures by adding some compatibility code
to `LoadBlockIndex` and changing `nTx` values from 1 to 0 when they are fake
(when `(pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_TRANSACTIONS`), but a
little simpler not to worry about being compatible in this case.

Add a test for a CheckBlockIndex crash that would happen before previous
"assumeutxo: Get rid of faked nTx and nChainTx values" commit.

The crash was an assert failure in the (pindex->nChainTx == pindex->nTx +
prev_chain_tx) check that would previously happen if a snapshot was loaded, and
a block was submitted which forked from the chain before the snapshot block and
after the last downloaded background chain block. This block would not be
marked assumed-valid because it would not be an ancestor of the snapshot, and
it would have nTx set, nChainTx unset, and prev->nChainTx set with a fake
value, so the assert would fail. After the fix, prev->nChainTx is unset instead
of being set to a fake value, so the assert succeeds. This test was originally
posted by maflcko in
bitcoin#29261 (comment)

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^[email protected]>

Add a test for a CheckBlockIndex crash that would happen before previous
"assumeutxo: Get rid of faked nTx and nChainTx values" commit.

The crash was an assert failure in the (pindex->nChainTx == pindex->nTx +
prev_chain_tx) check that would previously happen if the snapshot block was
submitted after loading the snapshot and downloading a few blocks after the
snapshot. In that case ReceivedBlockTransactions() previously would overwrite
the nChainTx value of the submitted snapshot block with a fake value based on
the previous block, so the (pindex->nChainTx == pindex->nTx + prev_chain_tx)
check would later fail on the first block after the snapshot. This test was
originally posted by Martin Zumsande <[email protected]> in
bitcoin#29370 (comment)

Co-authored-by: Martin Zumsande <[email protected]>

Flag adds complexity and is not currently used for anything.