Initialize base_value in pascal_object_print_value

The val_print removal series introduced a new possibly-uninitialized
warning in p-valprint.c.  Examination of the code shows that the
warning does not indicate a real bug, so this patch silences the
warning by setting the variable in the catch clause of a try/catch.
(The obvious initialization did not work due to a "goto" in this
function.)

gdb/ChangeLog
2020-03-16  Tom Tromey  <tom@tromey.com>

	* p-valprint.c (pascal_object_print_value): Initialize
	base_value.