patch 8.0.0502: Coverity complains about possible NULL pointer

Problem:    Coverity complains about possible NULL pointer.
Solution:   Add an assert(), let's see if this works on all systems.

Author: brammool

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    502,
 /**/
     501,
 /**/

src/window.c

@@ -3806,6 +3806,7 @@ close_tabpage(tabpage_T *tab)
 	for (ptp = first_tabpage; ptp != NULL && ptp->tp_next != tab;
 							    ptp = ptp->tp_next)
 	    ;
+	assert(ptp != NULL);
 	ptp->tp_next = tab->tp_next;
     }