wallet: Add missing cs_db lock

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.

src/wallet/db.cpp

@@ -147,6 +147,7 @@ BerkeleyEnvironment::BerkeleyEnvironment(const fs::path& dir_path) : strPath(dir
 
 BerkeleyEnvironment::~BerkeleyEnvironment()
 {
+    LOCK(cs_db);
     g_dbenvs.erase(strPath);
     Close();
 }