pr -e, with a mix of backspaces and TABs, could corrupt the heap

pr -e, with a mix of backspaces and TABs, could corrupt the heap

* tests/pr/Test.pm: New tests for the above.
* src/pr.c (char_to_clump): Ensure that "input_position" never
goes below 0.
Also, elide any backspace encountered when input_position is 0,
to be compatible at least with /bin/pr from Solaris 10.
This bug is present in the original version:
b25038ce9a234ea0906ddcbd8a0012e917e6c661
* NEWS [Bug fixes]: Mention this.
Report and diagnosis by Cristian Cadar, Daniel Dunbar and Dawson Engler
in http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/13272