Merge bitcoin#15322: wallet: Add missing cs_db lock

712d35b wallet: Add missing cs_db lock (João Barbosa)

Pull request description:

  Without this lock `BerkeleyEnvironment::~BerkeleyEnvironment` and `GetWalletEnv` would race for `g_dbenvs`. This wasn't detected before because thread safety analysis does not check constructors
  and destructors.

  Reference: http://releases.llvm.org/5.0.2/tools/clang/docs/ThreadSafetyAnalysis.html#no-checking-inside-constructors-and-destructors

Tree-SHA512: 350cb2b991ca699a6bca85f87c82c38f0814484c8ccb0d7d83cb3bff9afcf60dd32b2a9554a9e72eb5803bfad8b6970fe7da618b39be5889178b86faa1b74124

src/wallet/db.cpp

@@ -145,6 +145,7 @@ BerkeleyEnvironment::BerkeleyEnvironment(const fs::path& dir_path) : strPath(dir
 
 BerkeleyEnvironment::~BerkeleyEnvironment()
 {
+    LOCK(cs_db);
     g_dbenvs.erase(strPath);
     Close();
 }