Null check in g_hash_table_lookup()

[Michael McConville <mmcconv1 sccs swarthmore edu>]

Currently, if you pass a null key pointer to g_hash_table_lookup(), it
will sometimes segfault in g_str_hash(). This often happened in the
Pidgin development tip until we added this check:

        https://hg.pidgin.im/pidgin/main/rev/7e57fa4513f4

I was running GLib 2.44.1 on OpenBSD 5.8 (-current).

It seems like adding:

        g_return_val_if_fail (key != NULL, NULL);

Here:

        https://github.com/GNOME/glib/blob/master/glib/ghash.c#L1145

Should fix it. However, it seems saliently missing, so maybe I'm
misunderstanding and NULL can be a valid key pointer.