Assert threads are created in test 642026

We kept seeing the glib testsuite hanging on our CI system in the
testcase for 642026. After some digging it turned out the jenkins slave
was misconfigured and its task limit was too low.

Add an assertion that the test thread has been created to more easily
spot this error condition as opposed to the test simply hanging.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>

https://bugzilla.gnome.org/show_bug.cgi?id=769672

glib/tests/642026.c

@@ -64,6 +64,7 @@ testcase (void)
       freed = 0;
 
       t1 = g_thread_create (thread_func, NULL, TRUE, NULL);
+      g_assert (t1 != NULL);
 
       /* wait for t1 to set up its thread-private data */
       g_cond_wait (cond, mutex);