refactor: rpc: Pass CBlockIndex by reference instead of pointer

All functions assume that the pointer is never null, so pass by
reference, to avoid accidental segfaults at runtime, or at least make
them more obvious.

Also, remove unused c-style casts in touched lines.

Also, add CHECK_NONFATAL checks, to turn segfault crashes into an
recoverable runtime error with debug information.

src/test/blockchain_tests.cpp

@@ -41,7 +41,7 @@ static void RejectDifficultyMismatch(double difficulty, double expected_difficul
 static void TestDifficulty(uint32_t nbits, double expected_difficulty)
 {
     CBlockIndex* block_index = CreateBlockIndexWithNbits(nbits);
-    double difficulty = GetDifficulty(block_index);
+    double difficulty = GetDifficulty(*block_index);
     delete block_index;
 
     RejectDifficultyMismatch(difficulty, expected_difficulty);