Mike Fleetwood authored Feb 20, 2021 and Curtis Gedak committed Feb 22, 2021

Extract call to GLib's g_utf8_get_char_validated() and the associated
workaround to also read NUL characters into a separate function to make
PipeCapture::OnReadable() a little smaller and simpler, so easier to
understand.

Add max_len > 0 clause into get_utf8_char_validated() like this:
    if (uc == UTF8_PARTIAL && max_len > 0)
so that the NUL character reading workaround is only applied when
max_len specifies the maximum number of bytes to read, rather than
when -1 specifies reading a NUL termination string.  This makes
get_utf8_char_validated() a complete wrapper of
g_utf8_get_char_validated() [1], even though GParted always specifies
the maximum number of bytes to read.

No longer describe the inability to read NUL characters as a bug [2]
since the GLib author's said it wasn't [3].

[1] GLib Reference Manual, Unicode Manipulation Functions,
    g_utf8_get_char_validated ()
    https://developer.gnome.org/glib/stable/glib-Unicode-Manipu...