eager: Graceful failure on invalid inputs.

Tests added to pywrap_tfe_test.py would fail
(segmentation fault / infinite loop)
without corresponding fixes to pywrap_tfe.i and pywrap_tfe_src.cc

Other statements that would fail ungracefully without this fix
(and with eager execution enabled) include:
tf.split(value=0, num_or_size_splits=-1)
tf.dynamic_partition(data=0, partitions=0, num_partitions=-1)
tf.split(value=0, num_or_size_splits=1.23, num=-1)
tf.unstack(value=0, num=-1)

PiperOrigin-RevId: 212731927