libisu: properly handle nulls in isu_pkg_get_name/isu_pkg_get_version

libisu: properly handle nulls in isu_pkg_get_name/isu_pkg_get_version

This commit fixes the incorrect dereferencing of pointers when
retrieving data from the name/version members of the pkg_info
structure in the isu_pkg_get_name/isu_pkg_get_version functions
of the libisu public API.

Although a situation where an instance of the pkg_info structure is
correctly allocated, but its name/version components are NULL, should
not occur during normal usage, public functions must be resilient to
such malformed arguments and respond with an appropriate error rather
than a segfault.

Change-Id: I641b003568a0db79eab79f192c2f612bb6900721