THRIFT-3558 fix typos in c_glib test assertions

This closes #796
diff --git a/lib/c_glib/test/testoptionalrequired.c b/lib/c_glib/test/testoptionalrequired.c
index 60d8afd..818343c 100755
--- a/lib/c_glib/test/testoptionalrequired.c
+++ b/lib/c_glib/test/testoptionalrequired.c
@@ -79,7 +79,7 @@
   assert (s1->__isset_im_default == FALSE);
   assert (s1->__isset_im_optional == FALSE);  
   write_to_read (THRIFT_STRUCT (s1), THRIFT_STRUCT (s2), NULL, NULL);
-  assert (s2->__isset_im_default = TRUE);
+  assert (s2->__isset_im_default == TRUE);
   assert (s2->__isset_im_optional == FALSE);
   assert (s2->im_optional == 0);