sunrise-20 GLFW: Stop-gap to avoid crashing on non-printable keys

[cben]

[EDIT: I'm on Linux under wayland]
When trying sbcl --load src/bootstrap-glfw.lisp, key presses are already appended to box (even if not re-rendered but box is logged to stdout). But touching any key that is not alphanumeric tends to crash e.g.

debugger invoked on a SIMPLE-TYPE-ERROR in thread
#<THREAD tid=786576 "main thread" RUNNING {10010D81B3}>:
  Symbol name is not of length one: :MINUS

similarly, can't test non-nil mod-keys because touching a mod key crashes:

  Symbol name is not of length one: :LEFT-SHIFT

With this quick fix, you can at least go through the motions of modded keys:

glfw3 key-callback key3: LEFT-SHIFT scancode: 50 action: PRESS mod-keys: NIL input type: KEYWORD
  Error converting key to character: Symbol name is not of length one: :LEFT-SHIFT, ignoring.
  the initial box now: #<DATA-BOX H >
glfw3 key-callback key3: A scancode: 38 action: PRESS mod-keys: (SHIFT) input type: KEYWORD
  the initial box now: #<DATA-BOX HA >
glfw3 key-callback key3: A scancode: 38 action: RELEASE mod-keys: (SHIFT) input type: KEYWORD
"key-callback nothing"
glfw3 key-callback key3: LEFT-SHIFT scancode: 50 action: RELEASE mod-keys: (SHIFT) input type: KEYWORD

[Even if the mods are ignored; (CHARACTER :A) returns #\A either way. I suppose it'll get better once handle-boxer-input can be used...]