Fix segmentation fault on eval condition with no return value.

Signatures using an eval-condition that had no return value caused a
segmentation fault. This fix just returns false in this case, as it is
done for an interpreter error.

src/RuleCondition.cc

@@ -175,8 +175,13 @@ bool RuleConditionEval::DoMatch(Rule* rule, RuleEndpointState* state,
 	try
 		{
 		Val* val = id->ID_Val()->AsFunc()->Call(&args);
-		result = val->AsBool();
-		Unref(val);
+		if ( val )
+			{
+			result = val->AsBool();
+			Unref(val);
+			}
+		else
+			result = false;
 		}
 
 	catch ( InterpreterException& e )