__fish_cursor_xterm: Ignore unknown cursor settings

This prevents leaking the escape sequence by printing nonsense, and it
also allows disabling cursor setting by just setting the variable to
e.g. empty.

And if we ever added any shapes, it would allow them to be used on new
fish and ignored on old

Fixes #9698

(cherry picked from commit e45bddcbb1)

doc_src/interactive.rst

@@ -426,6 +426,8 @@ The ``fish_vi_cursor`` function will be used to change the cursor's shape depend
 
 Additionally, ``blink`` can be added after each of the cursor shape parameters to set a blinking cursor in the specified shape.
 
+Fish knows the shapes "block", "line" and "underscore", other values will be ignored.
+
 If the cursor shape does not appear to be changing after setting the above variables, it's likely your terminal emulator does not support the capabilities necessary to do this. It may also be the case, however, that ``fish_vi_cursor`` has not detected your terminal's features correctly (for example, if you are using ``tmux``). If this is the case, you can force ``fish_vi_cursor`` to set the cursor shape by setting ``$fish_vi_force_cursor`` in ``config.fish``. You'll have to restart fish for any changes to take effect. If cursor shape setting remains broken after this, it's almost certainly an issue with your terminal emulator, and not fish.
 
 .. _vi-mode-command:

share/functions/__fish_cursor_xterm.fish

@@ -8,6 +8,9 @@ function __fish_cursor_xterm -d 'Set cursor (xterm)'
             set shape 4
         case line
             set shape 6
+        case '*'
+            # Unknown shape
+            return
     end
     if contains blink $argv
         set shape (math $shape - 1)