vala: Don't use possibly uninitialized backing field of package_name
vala: Don't use possibly uninitialized backing field of package_name

Don't check _package_name directly, which may be null at the time
installed_version is accessed. This fixes a bug where Vala wouldn't
generate errors when using symbols that are unavailable for a package
version.

Introduced by 7f0e90a5c34f437b7ab8f9197a9d42fc835b1a60

Fixes https://gitlab.gnome.org/GNOME/vala/issues/971

frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala