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/gui_w32.c

@@ -7127,8 +7127,11 @@ dialog_callback(
 
 	    GetDlgItemTextW(hwnd, DLG_NONBUTTON_CONTROL + 2, wp, IOSIZE);
 	    p = utf16_to_enc(wp, NULL);
-	    vim_strncpy(s_textfield, p, IOSIZE);
-	    vim_free(p);
+	    if (p != NULL)
+	    {
+		vim_strncpy(s_textfield, p, IOSIZE);
+		vim_free(p);
+	    }
 	    vim_free(wp);
 	}