x11/keymap: be more defensive about the number of modifiers

There can be at most 16 vmods, and we rely on the facts that #vmods +
NUM_REAL_MODS (8) <= XKB_MAX_MODS (32) when accessing keymap->mods.mods.
But msb_pos() can potentially return up to #vmods = 32 if the server is
malicious, so we need to truncate it.

Signed-off-by: Ran Benita <ran234@gmail.com>