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/ex_cmds.c

@@ -3841,11 +3841,11 @@ ex_substitute(exarg_T *eap)
 	    which_pat = RE_LAST;	    // use last used regexp
 	    delimiter = *cmd++;		    // remember delimiter character
 	    pat = cmd;			    // remember start of search pat
-	    patlen = STRLEN(pat);
 	    cmd = skip_regexp_ex(cmd, delimiter, magic_isset(),
 							&eap->arg, NULL, NULL);
 	    if (cmd[0] == delimiter)	    // end delimiter found
 		*cmd++ = NUL;		    // replace it with a NUL
+	    patlen = STRLEN(pat);
 	}
 
 	/*