patch 9.1.1339: missing out-of-memory checks for enc_to_utf16()/utf16_to_enc()

Problem:  missing out-of-memory checks for enc_to_utf16() and
          utf16_to_enc()
Solution: Add out-of-memory checks and fix a few other minor issues
          (John Marriott)

This change does:
-  add missing out-of-memory checks for enc_to_utf16() and
   utf16_to_enc()
-  add a small optimisation in mch_errmsg_c() and mch_msg_c() (in
   message.c) to only call STRLEN() if needed.
-  fix a memory leak in winpty_term_and_job_init() (in terminal.c).

closes: #17191

Signed-off-by: John Marriott <basilisk@internode.on.net>
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 */
+/**/
+    1339,
 /**/
     1338,
 /**/