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.
diff --git a/src/version.c b/src/version.c
index ac1c67a..81b7486 100644
--- a/src/version.c
+++ b/src/version.c
@@ -768,6 +768,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1388,
+/**/
     1387,
 /**/
     1386,