Change CChain::Contains() to take reference

The `CChain::Contains()` method dereferences its input without checking,
potentially resulting in nullptr-dereference if invoked with `nullptr`.
To avoid this possibility, its input is changed to a reference instead.
Call sites are adapted accoringly, extra nullptr-check is added as
needed.

src/txmempool.cpp

@@ -45,7 +45,7 @@ bool TestLockPointValidity(CChain& active_chain, const LockPoints& lp)
     if (lp.maxInputBlock) {
         // Check whether active_chain is an extension of the block at which the LockPoints
         // calculation was valid.  If not LockPoints are no longer valid
-        if (!active_chain.Contains(lp.maxInputBlock)) {
+        if (!active_chain.Contains(*lp.maxInputBlock)) {
             return false;
         }
     }