Out-of-bound bug fix in common/config.cpp

Hi

     This is a small patch for a bug caught by gcc-4.2.1. The original
code references the address right before a string literal.  I believe
my fix is the right thing to do. :) Could someone confirm that this is
okay?

-Doug
===================================================
--- config.cpp.orig     2007-08-09 00:35:48.000000000 -0700
+++ config.cpp  2007-08-09 00:36:04.000000000 -0700
@@ -1122,7 +1122,7 @@
     : out(o), first(false)
   {
     first_prefix = _("# default: ");
-    num_blanks = strlen(first_prefix - 1);
+    num_blanks = strlen(first_prefix) - 1;
   }

   PosibErr<bool> ListDefaultDump::add(ParmStr d)