Bitcoin-Qt: fix crash on Windows caused by CDBEnv::EnvShutdown()

- can be triggerd by just adding -proxy=crashme with 0.7.1
- crash occured, when AppInit2() was left with return false; after the
  first call to bitdb.open() (Step 6 in init)
- this is caused by GetDataDir() or .string() in CDBEnv::EnvShutdown()
  called via the bitdb global destructor
- init fDbEnvInit and fMockDb to false in CDBEnv::CDBEnv()

src/db.h

@@ -33,6 +33,7 @@ class CDBEnv
 private:
     bool fDbEnvInit;
     bool fMockDb;
+    std::string strPath;
 
     void EnvShutdown();