There are two versions of WM_NORMAL_HINTS: the original pre-ICCCM
version (standardised by Xlib itself?) provides 15 elements of 32 bits
each, with the ICCCM v1 extending this by 3 additional elements.

Since the flags are enough to identify which elements are present, and
the structure is append-only, we only need to read the minimum length
between what the user provided and what we support.

Fixes a heap overrun found with ASan.

Signed-off-by: Daniel Stone <daniels@collabora.com>