wifi: mac80211: avoid out-of-bounds read for empty PREQ elements

In the Linux kernel, the following vulnerability has been resolved:

wifi: mac80211: avoid out-of-bounds read for empty PREQ elements

ieee80211_mesh_preq_size_ok() derives the location of the PREQ bottom
fields before checking whether the element contains even the fixed
header. ieee80211_mesh_hwmp_preq_get_bottom() reads the flags byte to
account for the optional Address Extension field. Consequently, an
empty PREQ element causes a one-byte read beyond its declared payload.

Move the helper call after both size checks, so the bottom fields are
only accessed when they are present.