Check for negative argument in Type.template_argument

Check for negative argument in Type.template_argument

typy_template_argument did not check if the template argument was
non-negative.  A negative value could cause a gdb crash.

2018-09-23  Tom Tromey  <tom@tromey.com>

	PR python/17284:
	* python/py-type.c (typy_template_argument): Check for negative
	argument number.

gdb/testsuite/ChangeLog
2018-09-23  Tom Tromey  <tom@tromey.com>

	PR python/17284:
	* gdb.python/py-template.exp (test_template_arg): Add test for
	negative template argument number.