fuzz: Avoid initializing version to less than MIN_PEER_PROTO_VERSION #20995

This fixes a fuzz bug introduced in #20881. Previously the nodes in the fuzz tests had their version initialized to a constant (PROTOCOL_VERSION). After #20881, the nodes have their version initialized to an arbitrary signed integer. This is problematic for several reasons:

• Both nVersion and m_greatest_common_version may be initialized to 0. If a version message is processed, this leads to a crash, because m_greatest_common_version must be INIT_PROTO_VERSION while the version message is processed. See #20138
• The "valid" range for nVersion is [MIN_PEER_PROTO_VERSION, std::numeric_limits<int32_t>::max()] (see check in net_processing)
• The "valid" range for m_greatest_common_version is std::min(nVersion, PROTOCOL_VERSION) (see net_processing)

Fix all issues by initializing nVersion and m_greatest_common_version to their valid ranges.

The crashers, if someone wants to try this at home:

( echo 'dmVyc2lvbgAWFhYWFhYWFhYWFhYWFhYWFhYWFhZp/29uAPX//xYWFhYWFhYWFhYWFhYWFhYWFhYW
FhYWFhYWaW9uAOr1//8WFhYWFha0ZXJzaW9uAPX//wAAAAAAABAAAAAAAAAAAAC0ZXJzaW9uAPX/
/wBPT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT09PT08AAAAAABAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAACgAAAAAAAAAAgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAB2ZXJzaW9uAACDJIO9vXYKAAAAAAAAAAAAAAAAAAAAAAB2ZfS1qmu1qhUVFWs=' | base64 --decode > /tmp/a ) && FUZZ=process_message_version ./src/test/fuzz/fuzz /tmp/a

( echo 'dmVyc2lvbgD//wAhTmiqN///NDcAAACENDL/iv//8DYAAHL///////79/RtcAJqamhqa/QEAAAD/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' | base64 --decode > /tmp/b ) && FUZZ=process_message_version ./src/test/fuzz/fuzz /tmp/b

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #20758 (net-processing refactoring -- lose globals, move implementation details from .h to .cpp by ajtowns)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

cr ACK fad3d7625aa1c2b6c343946e709e87e7168f9d9d

Thanks for fixing this: I keep hitting this assertion failure all the time :)