gdb/python: exception trying to create empty array

The following python command fails:

    (gdb) python print gdb.lookup_type('char').array(1, 0)
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    ValueError: Array length must not be negative
    Error while executing Python code.

The above is trying to create an empty array, which is fairly command
in Ada.

gdb/ChangeLog:

        * python/py-type.c (typy_array_1): Do not raise negative-length
        exception if N2 is equal to N1 - 1.

gdb/testsuite/ChangeLog:

        * gdb.python/py-type.exp: Add a couple test about empty
        array creation, and negative-length array creation.

gdb/python/py-type.c

@@ -528,7 +528,7 @@ typy_array_1 (PyObject *self, PyObject *args, int is_vector)
       n1 = 0;
     }
 
-  if (n2 < n1)
+  if (n2 < n1 - 1)
     {
       PyErr_SetString (PyExc_ValueError,
 		       _("Array length must not be negative"));