Prevent grabKey from accidentally grabbing all unbound keys

grabKeys doesn't check that a KeySym is valid before looking up the
KeyCode(s) it's bound to. In particular, KeySym 0 (NoSymbol) gets mapped to every
unbound KeyCode, since that's what XKeycodeToKeysym returns for those.

This can most easily be reproduced using `statusBar` in xmonad-contrib,
with def as the key-mapping function; this unexpectedly invokes the
following instances:

    instance Default b => a -> b where def = const def
    instance (Default a, Default b) => (a, b) where def = (def, def)
    instance Default CInt where def = 0

thus producing a function which binds `toggleStruts` to the (KeyMask,
KeySym) pair (0, 0) (and demonstrating why Default is a dangerous
abstraction). The person who reported this used xmodmap to clear KeySyms
from their numpad keys, and then xmonad would bind strut toggling to the
entire numpad.

Note that it is not reliably possible to override `def` in this
situation (without introducing a new type); the only thing we can do is
try to avoid the aftermath, and I would expect that an inadvertent 0
KeySym is the most common error here anyway.

Fixes: #293
Fixes: 40cb12c ("Grab all keycodes linked to each keysym, not just one")
Co-authored-by: Brandon S Allbery KF8NH <allbery.b@gmail.com>

src/XMonad/Main.hs

@@ -473,8 +473,11 @@ grabKeys = do
     -- build a map from keysyms to lists of keysyms (doing what
     -- XGetKeyboardMapping would do if the X11 package bound it)
     syms <- forM allCodes $ \code -> io (keycodeToKeysym dpy code 0)
-    let keysymMap = M.fromListWith (++) (zip syms [[code] | code <- allCodes])
-        keysymToKeycodes sym = M.findWithDefault [] sym keysymMap
+    let keysymMap' = M.fromListWith (++) (zip syms [[code] | code <- allCodes])
+    -- keycodeToKeysym returns noSymbol for all unbound keycodes, and we don't
+    -- want to grab those whenever someone accidentally uses def :: KeySym
+    let keysymMap = M.delete noSymbol keysymMap'
+    let keysymToKeycodes sym = M.findWithDefault [] sym keysymMap
     forM_ (M.keys ks) $ \(mask,sym) ->
          forM_ (keysymToKeycodes sym) $ \kc ->
               mapM_ (grab kc . (mask .|.)) =<< extraModifiers