tests: utils: Fix out-of-bounds access to memory in virTestCompareToFile
tests: utils: Fix out-of-bounds access to memory in virTestCompareToFile

Similarly to 3506f1ecfde, the contents of 'cmpcontent' may be an empty
string so the following code would access memory out of the array.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>

libvirt/libvirt,zippy2/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,eskultety/libvirt,crobinso/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,andreabolognani/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jardasgit/libvirt,crobinso/libvirt,eskultety/libvirt,fabianfreyer/libvirt,olafhering/libvirt,crobinso/libvirt,eskultety/libvirt,eskultety/libvirt,libvirt/libvirt,nertpinx/libvirt,zippy2/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,jfehlig/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,olafhering/libvirt,eskultety/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,andreabolognani/libvirt,nertpinx/libvirt,jardasgit/libvirt,jardasgit/libvirt