inflow.c: Do not leak tty.

In a case open() returns 0 tty might be leaked. While 0 should be
stdin (and therefore is an unlikely return value from open()), it's
still the case that the test should be for non-negative return values
from open().

gdb/ChangeLog:

2021-05-11 Alexandra Hájková <ahajkova@redhat.com>

	* inflow.c (new_tty): Do not leak tty.