fix curses.newwin() crash on narrow terminals in TUI dialogs

show_confirm() sized its window purely from message content, with no
upper bound — a long line (the Cryptgeon password URL in the
post-issuance "send email?" prompt) could easily exceed a narrow
terminal's width (e.g. a serial console), making curses.newwin()
raise "curses function returned NULL" and crash the whole app.
show_cert_form() had the same unguarded newwin() call, just with a
fixed 13x62 size instead of content-driven.

Clamp both windows' height/width to the actual screen size, and wrap
newwin() itself in try/except as a last-resort fallback (declined
confirm / cancelled form) in case clamping still isn't enough.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

openvpncertupdate.py

@@ -651,9 +651,15 @@ def show_confirm(stdscr, message: str) -> bool:
     """Y/N modal dialog. Returns True if user presses y/Y."""
     sh, sw = stdscr.getmaxyx()
     lines  = message.splitlines()
-    h = len(lines) + 6
-    w = max(max(len(l) for l in lines) + 6, 38)
-    win = curses.newwin(h, w, (sh - h) // 2, (sw - w) // 2)
+    # Clamp to the screen: an unbounded width (e.g. a long Cryptgeon URL line)
+    # makes curses.newwin() raise "curses function returned NULL" outright on
+    # a narrow terminal (e.g. a serial console) instead of just wrapping badly.
+    h = min(len(lines) + 6, sh)
+    w = min(max(max(len(l) for l in lines) + 6, 38), sw)
+    try:
+        win = curses.newwin(h, w, max(0, (sh - h) // 2), max(0, (sw - w) // 2))
+    except curses.error:
+        return False
     win.keypad(True)
     draw_box(win, "Confirm")
     for i, line in enumerate(lines):
@@ -704,8 +710,14 @@ def show_cert_form(
     Tab / Shift-Tab / Up / Down cycle fields and buttons. F5 regenerates password.
     Enter or Space activates the focused button. Ctrl-G confirms immediately. Escape cancels."""
     sh, sw = stdscr.getmaxyx()
-    h, w   = 13, 62
-    win    = curses.newwin(h, w, (sh - h) // 2, (sw - w) // 2)
+    # Clamp to the screen so curses.newwin() can't raise "curses function
+    # returned NULL" outright on a terminal narrower/shorter than the usual
+    # 80x24 (e.g. a serial console with an unusually small viewport).
+    h, w   = min(13, sh), min(62, sw)
+    try:
+        win = curses.newwin(h, w, max(0, (sh - h) // 2), max(0, (sw - w) // 2))
+    except curses.error:
+        return CertFormResult(cn=cn, email="", password="", confirmed=False)
     win.keypad(True)
     fw = w - 6   # field width