patch 8.1.0868: crash if triggering garbage collector after a function call

Problem:    Crash if triggering garbage collector after a function call.
            (Michael Henry)
Solution:   Don't call the garbage collector right away, do it later.
            (closes #3894)

src/userfunc.c

@@ -679,10 +679,10 @@ cleanup_function_call(funccall_T *fc)
 	{
 	    // We have made a lot of copies.  This can happen when
 	    // repetitively calling a function that creates a reference to
-	    // itself somehow.  Call the garbage collector here to avoid using
+	    // itself somehow.  Call the garbage collector soon to avoid using
 	    // too much memory.
 	    made_copy = 0;
-	    (void)garbage_collect(FALSE);
+	    want_garbage_collect = TRUE;
 	}
     }
 }

src/version.c

@@ -783,6 +783,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    868,
 /**/
     867,
 /**/