Merge #20995: fuzz: Avoid initializing version to less than MIN_PEER_PROTO_VERSION

fad3d7625aa1c2b6c343946e709e87e7168f9d9d fuzz: Avoid initializing version to less than MIN_PEER_PROTO_VERSION (MarcoFalke)
fa99e33aebed0109630474e11183b0726b410c2e fuzz: move-only FillNode implementation to cpp file (MarcoFalke)

Pull request description:

  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/
  ///+/f1oZWFkZXJzAAAAAM8BAAAAIAYibkYRGgtZyq8SaGVhZGVycwAAAAD/NDcAAACENDL/iv//
  8DYAAHL///////79/RtcAJqamhqa/QEAAAD////+/f1oZWFkZXJzAAAAAM8BAAAAIAYibkYRGgtZ
  yq8SaGVhZGVycwAAAADPAQAAACAGIm5GERoLWS1wb3J061u/KMNPOkwFXqZ///b5IgIAAD+5ubkb
  XD5hZGRyAJqamhqasP0BAAAAAAAAAP0BAAAAIf39/R0dHQAAAAAAMgAA///7//+gXqZ///b5IgIA
  AD+5ubm5ubm5AAAAAAAAAAAAAAAAAAAAAAAAAACAAAAAAAAAFgAAAAAAAAAAAAlBmv39/f1/f39B
  f39hZGRyAG5vAACaLgAdGzY2zwEAAAAgBiJuRhEaC1ktcG9ydOtbvyjDTzpMBV6mf//2+SICAAA/
  ubm5G1w+YWRkcgCampoamrD9AQAAAAAAAAD9AQAAACH9/f0dHR0AAAAAADIAAP//+///oF6mf//2
  +SICAAA/ubm5ubm5uQAAAAAAAAAAAAAAAAAAAAAAAAAAgAAAAAAAABYAAAAAAAAAAAAJQZr9/f39
  f39/QX9/YWRkcgBubwAAmi4AHRs2NjY2NjY2NjYCAgI2NgIA/f39/f39Nv39/TUmABxc' | base64 --decode > /tmp/b ) && FUZZ=process_message_version ./src/test/fuzz/fuzz /tmp/b
  ```

ACKs for top commit:
  practicalswift:
    cr ACK fad3d7625aa1c2b6c343946e709e87e7168f9d9d

Tree-SHA512: ea64ee99b94d8e619e3949d2d21252c1236412c0e40f44f2b73595ca70cd2da0bdab005fb1a54f65fb291e7b07fdd33577ce4a3a078ca933246b511ebcb0e52a

0 files changed, 0 insertions, 0 deletions