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(-)