GSocket missing calls to check_socket()

Submitted by Michael Catanzaro @mcatanzaro

Link to original bug (#793343)

Description

From bug #726375:

""" g_socket_get_credentials() and g_socket_create_source() were missing the check.

Adding the checks to get/set_options and get_available_bytes() caused a bunch of unit tests to fail. I wouldn't be suprised if that indicated that the unit tests are broken somehow, but I haven't had time to investigate. """

Dan suggests that g_socket_create_source() should return a source that immediately triggers an error condition in this case.

Version: 2.55.x