Make pandas data-sharing check compatible with numpy 1.7

NB. Using NumPy 1.7 results in one less array object in the base.base.... trail for a DataFrame.

This change makes the assertion cope with any length of base references, so it works with both 1.6 and 1.7.

Fixes:
 - iris.tests.test_pandas.TestAsDataFrame
  - test_copy_float_false
  - test_copy_int32_false
  - test_copy_int64_false