array.c: fix array size

* array.c (rb_ary_permutation): `p` is the array of size `r`, as
  commented at permute0().  since `n >= r` here, buffer overflow
  never happened, just reduce unnecessary allocation though.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46416 b2dd03c8-39d4-4d8f-98ff-823fe69b080e