Fix two bugs in gdbserver thread name handling

Simon pointed out that my earlier patch to gdbserver's thread name
code:

    commit 07b3255c3bae7126a0d679f957788560351eb236
    Author: Tom Tromey <tom@tromey.com>
    Date:   Thu Jul 13 17:28:48 2023 -0600

	Filter invalid encodings from Linux thread names

... introduced a regression.  This bug was that the iconv output was
not \0-terminated.

Looking at it, I found another bug as well -- replace_non_ascii would
not \0-terminate, and also would return the wrong pointer

This patch fixes both of them.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31153