patch 9.1.0692: Wrong patlen value in ex_substitute()

Problem:  Wrong patlen value in ex_substitute() (after 9.1.0426).
Solution: Compute patlen after finding end separator.
          (zeertzjq)

Add a more explicit test.  The test already passes as the only case
where a overlarge patlen value matters was fixed by patch 9.1.0689.

closes: #15565

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    692,
 /**/
     691,
 /**/