c++: beware of short ranges for state numbers

Now that we use small integral types, possibly unsigned (e.g.,
unsigned char), to store state numbers, using -1 to denote an empty
state (i.e., a state that stores no semantical value) is very
dangerous: it will be confused with state 255, which might be
non-empty.

Rather than allocating a larger range of state numbers to keep the
empty-state apart, let's use the number of a state known to store no
value.  The initial state, numbered 0, seems to fit perfectly the job.

Reported by Frank Heckenbach.
https://lists.gnu.org/archive/html/bug-bison/2019-11/msg00016.html

* data/skeletons/lalr1.cc (empty_state): Be 0.

data/skeletons/lalr1.cc

@@ -335,7 +335,8 @@ m4_define([b4_shared_declarations],
       symbol_number_type type_get () const YY_NOEXCEPT;
 
       /// The state number used to denote an empty symbol.
-      enum { empty_state = -1 };
+      /// We use the initial state, as it does not have a value.
+      enum { empty_state = 0 };
 
       /// The state.
       /// \a empty when empty.