add a pair of shorthand macros to validate an index into an array

Two variations:

IndexOk(idx, array)       validate that idx is a valid index into the array

IndexOkT(idx, array)      validate that idx is a valid index into the
                          array, excluding the final Terminator element

src/timeout.c

@@ -112,7 +112,7 @@ static const struct propname {
 const char *
 property_by_index(int idx, int *propertynum)
 {
-    if (!(idx >= 0 && idx < SIZE(propertynames) - 1))
+    if (!IndexOkT(idx, propertynames))
         idx = SIZE(propertynames) - 1;
 
     if (propertynum)