guile: Type-check the argument to 'history-append!'.

gdb/
2014-06-05  Ludovic Courtès  <ludo@gnu.org>

	* guile/scm-value.c (gdbscm_history_append_x): Use
	'vlscm_get_value_smob_arg_unsafe' instead of
	'vlscm_scm_to_value'.

gdb/testsuite/
2014-06-05  Ludovic Courtès  <ludo@gnu.org>

	* gdb.guile/scm-value.exp (test_value_in_inferior): Add test
	"history-append! type error".