Fix a few "Uninitialized scalar field" warnings

Fix a few warnings reported by Coverity.
None of these is critical, but making sure that class fields are
initialized can avoid heisenbugs.

src/db.cpp

@@ -227,10 +227,10 @@ CDB::CDB(const char *pszFile, const char* pszMode) :
     pdb(NULL), activeTxn(NULL)
 {
     int ret;
+    fReadOnly = (!strchr(pszMode, '+') && !strchr(pszMode, 'w'));
     if (pszFile == NULL)
         return;
 
-    fReadOnly = (!strchr(pszMode, '+') && !strchr(pszMode, 'w'));
     bool fCreate = strchr(pszMode, 'c');
     unsigned int nFlags = DB_THREAD;
     if (fCreate)