fix "no viable conversion from ... to 'bool'"

src/account.h

@@ -261,7 +261,7 @@ public:
   mutable optional<xdata_t> xdata_;
 
   bool has_xdata() const {
-    return xdata_;
+    return xdata_ != NULL;
   }
   void clear_xdata();
   xdata_t& xdata() {

src/item.h

@@ -191,7 +191,7 @@ public:
   static bool use_aux_date;
 
   virtual bool has_date() const {
-    return _date;
+    return _date != NULL;
   }
 
   virtual date_t date() const {

src/post.h

@@ -205,7 +205,7 @@ public:
   mutable optional<xdata_t> xdata_;
 
   bool has_xdata() const {
-    return xdata_;
+    return xdata_ != NULL;
   }
   void clear_xdata() {
     xdata_ = none;

src/times.h

@@ -568,7 +568,7 @@ public:
   void   stabilize(const optional<date_t>& date = none);
 
   bool   is_valid() const {
-    return start;
+    return start != NULL;
   }
 
   /** Find the current or next period containing date.  Returns false if