st.c (rb_hash_bulk_insert_into_st_table): avoid out-of-bounds write

	"hash_bulk_insert" first expands the table, but the target size was
	wrong: it was calculated by "num_entries + (size to buld insert)", but
	it was wrong when "num_entries < entries_bound", i.e., it has a deleted
	entry.  "hash_bulk_insert" adds the given entries from entries_bound,
	which led to out-of-bounds write access.  [Bug #15536]

	As a simple fix, this commit changes the calculation to "entries_bound +
	size".  I'm afraid if this might be inefficient, but I think it is safe
	anyway.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66853 b2dd03c8-39d4-4d8f-98ff-823fe69b080e