a11y: Fix offset handling for AT-SPI GetCharacterAtOffset

The text retrieved using `gtk_accessible_text_get_contents`
already contains only the character at the given offset,
and so the character is at index 0 in `str`, rather than at
the same offset again, so adjust this accordingly.

With this in place, querying the character in a
LibreOffice paragraph consisting of the text
"Hello world." now gives the expected results
with a pending LibreOffice change [1] to support
the new GtkAccessibleText interface:

    In [1]: text  = acc.queryText()
    In [2]: text.getCharacterAtOffset(0)
    Out[2]: 72
    In [3]: text.getCharacterAtOffset(1)
    Out[3]: 101
    In [4]: text.getCharacterAtOffset(2)
    Out[4]: 108
    In [5]: text.getCharacterAtOffset(3)
    Out[5]: 108
    In [6]: text.getCharacterAtOffset(4)
    Out[6]: 111

Previously, this would only work correctly
for an index of 0:

    In [1]: text = acc.queryText()
    In [2]: text.getCharacterAtOffset(0)
    Out[2]: 72
    In [3]: text.getCharacterAtOffset(1)
    Out[3]: 0
    In [4]: text.getCharacterAtOffset(2)
    Out[4]: 0
    In [5]: text.getCharacterAtOffset(3)
    Out[5]: 0
    In [6]: text.getCharacterAtOffset(4)
    Out[6]: 0

[1] https://gerrit.libreoffice.org/c/core/+/163733

gtk/a11y/gtkatspitext.c

@@ -130,9 +130,8 @@ accessible_text_handle_method (GDBusConnection       *connection,
       if (text != NULL)
         {
           const char *str = g_bytes_get_data (text, NULL);
-
-          if (0 <= offset && offset < g_utf8_strlen (str, -1))
-            ch = g_utf8_get_char (g_utf8_offset_to_pointer (str, offset));
+          if (g_utf8_strlen (str, -1) > 0)
+            ch = g_utf8_get_char (str);
         }
 
       g_dbus_method_invocation_return_value (invocation, g_variant_new ("(i)", ch));