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-Manipulation.html#g-utf8-get-char-validated

[2] 8dbbb47c
    Workaround g_utf8_get_char_validate() bug with embedded NUL bytes
    (#777973)

[3] Bug 780095 - g_utf8_get_char_validated() stopping at nul byte even
    for length specified buffers
    https://bugzilla.gnome.org/show_bug.cgi?id=780095#18
        "If g_utf8_get_char_validated() encounters a nul byte in the
        middle of a string of given longer length, it returns -2,
        indicating a partial gunichar.  That is not the obvious
        behaviour, but since g_utf8_get_char_validated() has been API
        for a long time, the behaviour cannot be changed.
        "

Closes #136 - 1.2.0: test suite is failing in test_PipeCapture