dpkg-gensymbols: warn when the generated symbols files is empty

It indirectly avoids a perl warning in dpkg-gensymbols when no symbols
file has been generated (because it would have been empty).

Reported-by: Niels Thykier <niels@thykier.net>

debian/changelog

@@ -19,6 +19,8 @@ dpkg (1.16.1) UNRELEASED; urgency=low
   * "any" no longer hides "all" in the Architecture field of a .dsc.
   * Fix dpkg --remove to really remove the triggers from the various
     internal files in /var/lib/dpkg/info/triggers/. Closes: #525160
+  * Avoid a perl warning in dpkg-gensymbols when no symbols file has been
+    generated (because it would have been empty). Closes: #626684
 
   [ Guillem Jover ]
   * Install deb-src-control(5) man pages in dpkg-dev. Closes: #620520

scripts/dpkg-gensymbols.pl

@@ -287,7 +287,9 @@ unless ($quiet) {
     $md5_after->addfile($after);
     # Output diffs between symbols files if any
     if ($md5_before->hexdigest() ne $md5_after->hexdigest()) {
-	if (defined($ref_symfile->{file})) {
+	if (not defined($output)) {
+	    warning(_g("the generated symbols file is empty"));
+	} elsif (defined($ref_symfile->{file})) {
 	    warning(_g("%s doesn't match completely %s"),
 		    $output, $ref_symfile->{file});
 	} else {