Fixes SCREEN_HARDSTATUS default value

If SCREEN_HARDSTATUS is null, default it to "", not ":".

Change-Id: I52d4eb6bb494cca995e33971d5f1eec953d484fd
diff --git a/stack.sh b/stack.sh
index aeaaf60..4eed4ca 100755
--- a/stack.sh
+++ b/stack.sh
@@ -916,7 +916,7 @@
     sleep 1
 
     # Set a reasonable status bar
-    SCREEN_HARDSTATUS=${SCREEN_HARDSTATUS-:}
+    SCREEN_HARDSTATUS=${SCREEN_HARDSTATUS:-}
     if [ -z "$SCREEN_HARDSTATUS" ]; then
         SCREEN_HARDSTATUS='%{= .} %-Lw%{= .}%> %n%f %t*%{= .}%+Lw%< %-=%{g}(%{d}%H/%l%{g})'
     fi