Don't store inactive timers in the saved game

Thanks to @fosnola for spotting this.

src/scenario/special.cpp

@@ -22,6 +22,12 @@
 #include "damage.hpp"
 #include "fields.hpp"
 
+bool cTimer::is_valid() const {
+	if(time < 0) return false;
+	if(node < 0) return false;
+	return true;
+}
+
 cSpecial::cSpecial(){
 	type = eSpecType::NONE;
 	sd1 = -1;

src/scenario/special.hpp

@@ -98,6 +98,7 @@ public:
 	long time = 0;
 	eSpecCtxType node_type = eSpecCtxType::SCEN;
 	short node = -1;
+	bool is_valid() const;
 };
 
 // HAIL means called when initiating conversation.