trie: fix a temporary memory leak in the memcache

[karalabe]

This PR is a 3 liner fix. The rest is just a repro/verification.

We've received from time to time reports that Geth closes with ERROR[...] Dangling trie nodes after full cleanup. We've been hacking on the memcache for quite a lot, so I'm unsure if the old issues are the same as the one this PR fixes, but this one addesses specifically an issue where certain nodes remain in the memory cache even though they have no more references left.

The issue happens when a trie node exists on disk (never loaded or already committed), and it's recreated by the trie again (short node split into full, and them merged back into short). This will cause the node to be inserted into the memcache, with a potentially invalid parent count (0). Dereferencing this trie node form memory will first overflow its counter to MAXINT, thus never cleaning it out.

The node still remains part of the flush-list, so it will eventually be pushed out to disk, but in the mean time it's a dangling node. The PR fixes it by adding an explicit check for the 0-parent case.

[fjl]

I don't understand how a node with zero parents can get into the Database, but the fix looks OK. Maybe the root cause is a missing dereference call?

[karalabe]

The node is already in the database, and the same node is recreated (not loaded). That will result in it being inserted into the memcache again.

[emile]

I'm trying out Streaming chain tracers but get an error on any commit from 67a7857 onwards when this fix was merged:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x69cb7e]

goroutine 130 [running]:
github.com/ethereum/go-ethereum/trie.(*Database).reference(...)
        github.com/ethereum/go-ethereum/trie/database.go:423
github.com/ethereum/go-ethereum/trie.(*Database).Reference(0xc420648870, 0x64044303454d7ed6, 0x855343477142ae25, 0xe6dda1c0db60b87a, 0xf32b533acdb5414b, 0x0, 0x0, 0x0, 0x0)
        github.com/ethereum/go-ethereum/trie/database.go:412 +0x18e
github.com/ethereum/go-ethereum/eth.(*PrivateDebugAPI).traceChain.func2(0xc42008fc80, 0xc42020a890, 0xc42032a1b0, 0xc4205ce240, 0xbecc99cb06ef0fd6, 0x1e8ead4f0, 0x1b20400, 0xc42008fce0, 0xc42055c4c0, 0x0, ...)
        github.com/ethereum/go-ethereum/eth/api_tracer.go:295 +0x894
created by github.com/ethereum/go-ethereum/eth.(*PrivateDebugAPI).traceChain
        github.com/ethereum/go-ethereum/eth/api_tracer.go:220 +0x698