dpkg-buildpackage: Detect missing sign-command before starting the build

While a failure in signing the resulting built artifacts is something
that can be easily recovered from, by just signing them again, it's
better if we just catch this beforehand.

debian/changelog

@@ -7,6 +7,8 @@ dpkg (1.17.6) UNRELEASED; urgency=low
     specify a package checker to use before the signing process.
   * Detect a missing gain-root-command even if dpkg-buildpackage is running
     as root.
+  * Detect a missing sign-command in dpkg-buildpackage, before starting the
+    build, to avoid a failure at the end of the process.
 
   [ Updated scripts translations ]
   * German (Helge Kreutzmann).

scripts/dpkg-buildpackage.pl

@@ -313,9 +313,12 @@ if ($check_command and not find_command($check_command)) {
     error(_g("check-commmand '%s' not found"), $check_command);
 }
 
-if (!defined $signcommand &&
-    (($ENV{GNUPGHOME} && -e $ENV{GNUPGHOME}) ||
-     ($ENV{HOME} && -e "$ENV{HOME}/.gnupg"))) {
+if ($signcommand) {
+    if (!find_command($signcommand)) {
+        error(_g("sign-commmand '%s' not found"), $signcommand);
+    }
+} elsif (($ENV{GNUPGHOME} && -e $ENV{GNUPGHOME}) ||
+         ($ENV{HOME} && -e "$ENV{HOME}/.gnupg")) {
     if (find_command('gpg2')) {
         $signcommand = 'gpg2';
     } elsif (find_command('gpg')) {