Avoid overflow in arena_run_regind().

Fix a regression due to:
    Remove an arena_bin_run_size_calc() constraint.
    2a6f2af6
The removed constraint required that small run headers fit in one page,
which indirectly limited runs such that they would not cause overflow in
arena_run_regind().  Add an explicit constraint to
arena_bin_run_size_calc() based on the largest number of regions that
arena_run_regind() can handle (2^11 as currently configured).