patch 8.1.1388: errors when calling prop_remove() for an unloaded buffer

Problem:    Errors when calling prop_remove() for an unloaded buffer.
Solution:   Bail out when the buffer is not loaded.  Add a few more tests for
            failing when the buffer number is invalid.

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1388,
 /**/
     1387,
 /**/