Exit gracefully on invalid --gtest_* flag.

My gtest run crashed when I provided wrong flag (e.g. --gtest_list),
because it tried to call std::vector::back() on empty testcase_list.

Test: /data/nativetest/bionic-unit-tests/bionic-unit-tests works.
Test: `/d/n/b/bionic-unit-tests --gtest_invalid` dumps the gtest help
      message.
Change-Id: I5f63c7fa6476d8749248c9b54d3023446f3e0e77