alloc: Fix lvextend failure when varying stripes.

alloc: Fix lvextend failure when varying stripes.

A segfault was reported when extending an LV with a smaller number of
stripes than originally used.  Under unusual circumstances, the cling
detection code could successfully find a match against the excess
stripe positions and think it had finished prematurely leading to an
allocation being pursued with a length of zero.

Rename ix_offset to num_positional_areas and move it to struct
alloc_state so that _is_condition() can obtain access to it.

In _is_condition(), areas_size can no longer be assumed to match the
number of positional slots being filled so check this newly-exposed
num_positional_areas directly instead.  If the slot is outside the
range we are trying to fill, just ignore the match for now.

(Also note that the code still only performs cling detection against
the first segment of the LV.)