Merge bitcoin/bitcoin#24032: Add defaults to vDeployments to avoid uninitialized variables

c4c5b9c consensus/params: set default values for BIP9Deployment (Anthony Towns)

Pull request description:

  Adds default values for `vDeployments` in `consensus/params.h` so that undefined behaviour is avoided if a deployment is not initialized. Also adds a check in the unit tests to alert if this is happening, since even if it doesn't invoke undefined behaviour it's probably a mistake.

ACKs for top commit:
  laanwj:
    Code review ACK c4c5b9c

Tree-SHA512: 22d7ff86a817d9e9e67c47301fc3b7e9d5821c13565d7706711f113dea220eea29b413a7c8d029691009159cebc85a108d77cb52418734091c196bafb2b12423

Author: laanwj

src/consensus/params.h

@@ -40,11 +40,11 @@ constexpr bool ValidDeployment(DeploymentPos dep) { return dep < MAX_VERSION_BIT
  */
 struct BIP9Deployment {
     /** Bit position to select the particular bit in nVersion. */
-    int bit;
+    int bit{28};
     /** Start MedianTime for version bits miner confirmation. Can be a date in the past */
-    int64_t nStartTime;
+    int64_t nStartTime{NEVER_ACTIVE};
     /** Timeout/expiry MedianTime for the deployment attempt. */
-    int64_t nTimeout;
+    int64_t nTimeout{NEVER_ACTIVE};
     /** If lock in occurs, delay activation until at least this block
      *  height.  Note that activation will only occur on a retarget
      *  boundary.

src/test/versionbits_tests.cpp

@@ -273,6 +273,7 @@ static void check_computeblockversion(VersionBitsCache& versionbitscache, const
         nStartTime == Consensus::BIP9Deployment::NEVER_ACTIVE)
     {
         BOOST_CHECK_EQUAL(min_activation_height, 0);
+        BOOST_CHECK_EQUAL(nTimeout, Consensus::BIP9Deployment::NO_TIMEOUT);
         return;
     }