dpkg: Fix --verify with --root

The code was not taking into account the root prefix when verifying
pathnames, which resulted in all files failing to verify.

debian/changelog

@@ -12,6 +12,7 @@ dpkg (1.18.3) UNRELEASED; urgency=low
     command instead of executing itself with --set or --auto per subtask.
   * Add kfreebsd-armhf support to ostable and triplettable. Closes: #796283
     Thanks to Steven Chamberlain <steven@pyro.eu.org>.
+  * Fix «dpkg --verify» with --root.
   * Perl modules:
     - Only warn on invalid week days instead of aborting in
       Dpkg::Changelog::Entry::Debian. Regression introduced in dpkg 1.18.2.

src/verify.c

@@ -96,6 +96,7 @@ static void
 verify_package(struct pkginfo *pkg)
 {
 	struct fileinlist *file;
+	struct varbuf filename = VARBUF_INIT;
 
 	ensure_packagefiles_available(pkg);
 	parse_filehash(pkg, &pkg->installed);
@@ -116,9 +117,14 @@ verify_package(struct pkginfo *pkg)
 				fnn->newhash = fnn->oldhash;
 		}
 
+		varbuf_reset(&filename);
+		varbuf_add_str(&filename, instdir);
+		varbuf_add_str(&filename, fnn->name);
+		varbuf_end_str(&filename);
+
 		memset(&checks, 0, sizeof(checks));
 
-		md5hash(pkg, hash, fnn->name);
+		md5hash(pkg, hash, filename.buf);
 		if (strcmp(hash, fnn->newhash) != 0) {
 			checks.md5sum = VERIFY_FAIL;
 			failures++;