Only allow non-zero index adjustmetns

The current draft allows for an index adjustment of "0" which
creates ambiguity because "1/foo", "1+0/foo", and "1-0/foo"
are now three different ways to write a pointer with the exact
same effect.

This complicates round-trips between textual and functional
representations, the original text would need to be preserved
in order to re-constitute it correctly.  There is no need for
this added complication.

relative-json-pointer.xml

@@ -103,9 +103,10 @@
                         ; json-pointer from RFC 6901
 
   origin-specification  = non-negative-integer [ index-manipulation ]
-  index-manipulation    = ( "+" / "-" ) non-negative-integer
-  non-negative-integer  = "0" / %x31-39 *DIGIT
-                        ; zero, or digits without a leading zero
+  index-manipulation    = ( "+" / "-" ) positive-integer
+  non-negative-integer  = "0" / positive-integer
+  positive-integer      = %x31-39 *DIGIT
+                        ; digits without a leading zero
 ]]></sourcecode>
         </section>