Fix an abort in the tokenization of negative numbers.

src/json.json_lexer.m:
	Require that '-' be followed by a digit when tokenizing
	a number.  (The fact that this was not being checked for
	was leading to string.det_to_float/1 aborting later on.)

tests/zinc_soln_sep.{exp,json}:
	Add a regression test for the above.