Added division by zero detection

src/ee.h

@@ -1,5 +1,6 @@
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdbool.h>
 #include <math.h>
 #include <ctype.h>
 #include <string.h>
@@ -9,6 +10,7 @@ const int ERR_NS_OPERANDS       = 1;
 const int ERR_NS_OPERATORS      = 2;
 const int ERR_INVALID_INPUT     = 4;
 const int ERR_INVALID_FACTORIAL = 8;
+const int ERR_DIV_BY_ZERO       = 16;
 
 
 struct EE_STACK_NODE {