do not crash in a terminal < 76 columns wide

for https://bugs.launchpad.net/ubuntu-z-systems/+bug/1874314

DESIGN.md

@@ -27,9 +27,11 @@
     3. Make the common case as easy as possible by doing things like thinking
        about which widget should be highlighted when a screen is first shown.
 
-4. Subiquity is functional in an 80x24 terminal.  It doesn't matter if it falls
-   apart in a smaller terminal and obviously you can get more information on a
-   larger terminal at once, but it needs to work in 80x24.
+4. Subiquity is functional in an 80x24 terminal.  It doesn't matter if it
+   becomes unusable in a smaller terminal (although it shouldn't crash, as we
+   support people logging in via ssh and they can resize their terminal
+   however they like) and obviously you can get more information on a larger
+   terminal at once, but it needs to work in 80x24.
 
 ### urwid specific ranting
 

subiquitycore/ui/anchors.py

@@ -54,6 +54,9 @@ class HeaderColumns(Columns):
         center = max(int_scale(79, 101, maxcol + 1), 76)
         message = center - btn
         pad = (maxcol - center)//2
+        if pad <= 0:
+            pad = 1
+            message = maxcol - 2 - btn
         return [pad, message, btn, pad]