Merge #14834: validation: assert that pindexPrev is non-null when required

fbaaf782ce validation: assert that pindexPrev is non-null when required (Karl-Johan Alm)

Pull request description:

  In `ContextualCheckBlock`, we are checking if `pindexPrev == nullptr` conditionally at the start, but then assume it is non-`null` later. This removes the latter assumption.

Tree-SHA512: 95f1e9dc839b2cc0e099d155e6180634ece8c6760d00b53e7d27128762e64c92e82d98a5f4a5786b48a4851b17cdbb4b667d3b6a99adb651256e2032de67d05c

src/validation.cpp

@@ -3267,6 +3267,7 @@ static bool ContextualCheckBlock(const CBlock& block, CValidationState& state, c
     // Start enforcing BIP113 (Median Time Past) using versionbits logic.
     int nLockTimeFlags = 0;
     if (VersionBitsState(pindexPrev, consensusParams, Consensus::DEPLOYMENT_CSV, versionbitscache) == ThresholdState::ACTIVE) {
+        assert(pindexPrev != nullptr);
         nLockTimeFlags |= LOCKTIME_MEDIAN_TIME_PAST;
     }