libdpkg: Ignore not-installed packages for source related virtual fields

The source name and source versions are not available for not-installed
packages, so we should not try to print anything as that will end up
being bogus, or produce parsing problems for version related virtual
fields.

Closes: #972580

lib/dpkg/pkg-format.c

@@ -333,6 +333,9 @@ virt_source_package(struct varbuf *vb,
 	const char *name;
 	size_t len;
 
+	if (pkg->status == PKG_STAT_NOTINSTALLED)
+		return;
+
 	name = pkgbin->source;
 	if (name == NULL)
 		name = pkg->set->name;
@@ -347,6 +350,9 @@ virt_source_version(struct varbuf *vb,
                     const struct pkginfo *pkg, const struct pkgbin *pkgbin,
                     enum fwriteflags flags, const struct fieldinfo *fip)
 {
+	if (pkg->status == PKG_STAT_NOTINSTALLED)
+		return;
+
 	varbuf_add_source_version(vb, pkg, pkgbin);
 }
 
@@ -357,6 +363,9 @@ virt_source_upstream_version(struct varbuf *vb,
 {
 	struct dpkg_version version;
 
+	if (pkg->status == PKG_STAT_NOTINSTALLED)
+		return;
+
 	pkg_source_version(&version, pkg, pkgbin);
 
 	varbuf_add_str(vb, version.version);