avoid precedence warning

With Perl 5.20, installing a package with stow gives a warning like
this:

  Possible precedence issue with control flow operator at
  /gar/packages/stow-2.2.0/lib/perl5/site_perl/5.20.0/Stow.pm line 1736.

http://lists.gnu.org/archive/html/bug-stow/2014-06/msg00000.html

Suggested-by: Adam Sampson <ats@offog.org>

lib/Stow.pm.in

@@ -1734,8 +1734,8 @@ sub read_a_link {
     }
     elsif (-l $path) {
         debug(4, "  read_a_link($path): real link");
-        return readlink $path
-            or error("Could not read link: $path ($!)");
+        my $target = readlink $path or error("Could not read link: $path ($!)");
+        return $target;
     }
     internal_error("read_a_link() passed a non link path: $path\n");
 }