gutf8: add string length check when ending character offset is -1

Some function such as atk_text_get_text, use -1 to indicate the end of the
string. And an crash occurs when the -1 is passed to g_utf8_substring.

Call Trace:
  0  __memmove_avx_unaligned_erms
  1  memcpy
  2  g_utf8_substring
  3  impl_GetText
  4  handle_other
  5  handle_message
  6  _dbus_object_tree_dispatch_and_unlock
  7  dbus_connection_dispatch
  8  dbus_connection_dispatch
  9  ()
  10 g_main_dispatch
  11 g_main_context_dispatch
  12 g_main_context_iterate
  13 g_main_context_iteration
  14 g_application_run
  15 main

Signed-off-by: Chen Guanqiao <chen.chenchacha@foxmail.com>

glib/tests/utf8-misc.c

@@ -128,6 +128,10 @@ test_utf8_substring (void)
   r = g_utf8_substring ("abc\xe2\x82\xa0gh\xe2\x82\xa4", 2, 5);
   g_assert_cmpstr (r, ==, "c\xe2\x82\xa0g");
   g_free (r);
+
+  r = g_utf8_substring ("abcd", 1, -1);
+  g_assert_cmpstr (r, ==, "bcd");
+  g_free (r);
 }
 
 static void