lib/oe/recipeutils: fix patch_recipe*() with empty input
If you supplied an empty file to patch_recipe() (or an empty list to
patch_recipe_lines()) then the result was IndexError because the code
checking to see if it needed to add an extra line of padding didn't
check to see if there were in fact any lines before trying to access the
last line.
Fixes [YOCTO #9972].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
