st.c: Do not clear entries_bound when calling Hash#shift for empty
	 hash

	tab->entries_bound is used to check if the bins are full in
	rebuild_table_if_necessary.

	Hash#shift against an empty hash assigned 0 to tab->entries_bound, but
	didn't clear the bins. Thus, the table is not rebuilt even when the bins
	are full. Attempting to add a new element into full-bin hash gets stuck.

	This change stops clearing tab->entries_bound in Hash#shift.
	[Bug #18578]
	---
	 st.c                   |  1 -
	 test/ruby/test_hash.rb | 13 +++++++++++++
	 2 files changed, 13 insertions(+), 1 deletion(-)