libkmod: consider empty signature key as invalid

libkmod: consider empty signature key as invalid

A segmentation fault occurs if a module has an empty key attached to
its signature. This is mostly likely due to a corrupted module.

The crash happens because kmod_module_get_info() assumes that
kmod_module_signature_info() returns a signature of at least 1 byte.

The fix is based on a patch from Tobias Stoeckmann
<tobias@stoeckmann.org>, but rather than changing kmod_module_get_info()
to fix the crash, this changes kmod_module_signature_info() to
consider the signature as invalid.