xkbcomp: where a keysym cannot be resolved, set it to NoSymbol

xkbcomp: where a keysym cannot be resolved, set it to NoSymbol

Where resolve_keysym fails we warn but use the otherwise uninitialized variable
as our keysym. That later ends up in the keymap as random garbage hex value.

Simplest test case, set this in the 'us' keymap:
    key <TLDE>               {      [        xyz ] };

And without this patch we get random garbage:
./build/xkbcli-compile-keymap --layout us | grep TLDE:
    key <TLDE>               {      [      0x018a5cf0 ] };

With this patch, we now get NoSymbol:
./build/xkbcli-compile-keymap --layout us | grep TLDE:
    key <TLDE>               {      [        NoSymbol ] };