dpkg-vendor: Error out when no action option is provided

It was previously giving perl warnings due to uninitialized variables.

scripts/dpkg-vendor.pl

@@ -78,6 +78,8 @@ while (@ARGV) {
     }
 }
 
+usageerr(_g("need an action option")) unless defined($action);
+
 # Uses $ENV{DEB_VENDOR} if set
 $vendor = get_current_vendor() unless defined($vendor);