fix crash on shutdown when e.g. changing -txindex and abort action

- fixes #3136
- the problem is related to Boost path and a static initialized internal
  pointer
- using a std::string in CDBEnv::EnvShutdown() prevents the problem
- this removes the boost::filesystem::path path field from CDBEnv

src/wallet/db.h

@@ -27,7 +27,9 @@ class CDBEnv
 private:
     bool fDbEnvInit;
     bool fMockDb;
-    boost::filesystem::path path;
+    // Don't change into boost::filesystem::path, as that can result in
+    // shutdown problems/crashes caused by a static initialized internal pointer.
+    std::string strPath;
 
     void EnvShutdown();