Do not abort when there is no binary to compare in a package

Do not abort when there is no binary to compare in a package

When given a package which has no binary to compare, abipkgdiff
aborts.  This patch fixes that.

	* tests/data/test-diff-pkg/empty-pkg-libvirt-0.9.11.3-1.el7.ppc64.rpm:
	New input test package.
	* tests/data/test-diff-pkg/empty-pkg-libvirt-1.2.17-13.el7_2.2.ppc64.rpm:
	Likewise.
	* tests/data/test-diff-pkg/empty-pkg-report-0.txt: New test
	reference output.
	* data/Makefile.am: Add the new test material above to the build system.
	* tests/test-diff-pkg.cc (int_out_specs): Add the new test inputs
	to the set of tests.
	* tools/abipkgdiff.cc (compare): Do not abort if there is no
	binary to compare.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>