make GetFirstStoredBlock assert that 'start_block' always has data

And transfer the responsibility of verifying whether 'start_block'
has data or not to the caller.

This is because the 'GetFirstStoredBlock' function responsibility
is to return the first block containing data. And the current
implementation can return 'start_block' when it has no data!. Which
is misleading at least.

Edge case behavior change:
Previously, if the block tip lacked data but all preceding blocks
contained data, there was no prune violation. And now, such
scenario will result in a prune violation.

src/node/blockstorage.h

@@ -222,7 +222,9 @@ public:
     //! Returns last CBlockIndex* that is a checkpoint
     const CBlockIndex* GetLastCheckpoint(const CCheckpointData& data) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
-    //! Find the first block that is not pruned
+    //! Find the first stored ancestor of start_block immediately after the last
+    //! pruned ancestor. Return value will never be null. Caller is responsible
+    //! for ensuring that start_block has data is not pruned.
     const CBlockIndex* GetFirstStoredBlock(const CBlockIndex& start_block LIFETIMEBOUND) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     /** True if any block files have ever been pruned. */