libdpkg: Only ignore older packages if the existing version is informative

When ignoring older packages (either by explicit request, or when
parsing available files), we should not ignore them when the existing
version is empty, because then we would consider that the empty version
(equivalent to "0-0") is newer than "0~0-0" for example.

This affects any program using libdpkg to parse the available file.

Regression introduced in commit 86823d5792af8328107c3d86e486d2cf373da128.

Closes: #676664

debian/changelog

@@ -79,6 +79,9 @@ dpkg (1.17.0) UNRELEASED; urgency=low
   * Remove update-alternatives, dpkg-divert and dpkg-statoverride
     compatibility symlinks under /usr/sbin/.
   * Remove install-info wrapper.
+  * Only ignore older packages if the existing version is informative. This
+    allows any program using libdpkg to parse the available file to see again
+    packages with versions lesser than 0-0 (like 0~0-0). Closes: #676664
 
   [ Updated manpages translations ]
   * Fix incorrect translation of "fortify" in French manpage for dpkg-buildflags

lib/dpkg/parse.c

@@ -742,6 +742,7 @@ int parsedb(const char *filename, enum parsedbflags flags,
       db_pkgbin = &db_pkg->installed;
 
     if (((flags & pdb_ignoreolder) || ps.type == pdb_file_available) &&
+        dpkg_version_is_informative(&db_pkgbin->version) &&
         dpkg_version_compare(&new_pkgbin->version, &db_pkgbin->version) < 0)
       continue;