debian: Fix postinst error with no /var/lib/bitlbee

(Patch from debbug#816200, changelog modified slightly by dx)

debian/bitlbee-common.postinst

@@ -34,7 +34,9 @@ else
 	adduser --system --group --disabled-login --disabled-password --home /var/lib/bitlbee/ bitlbee
 fi
 
-chmod 700 /var/lib/bitlbee/
+if [ -d /var/lib/bitlbee ]; then
+    chmod 700 /var/lib/bitlbee/
+fi
 
 ## Can't do this in packaging phase: Don't know the UID yet. Access to
 ## the file should be limited, now that it stores passwords. Added

debian/changelog

@@ -1,8 +1,13 @@
 bitlbee (3.5.1-1.1) UNRELEASED; urgency=medium
 
+  [ dequis ]
   * Remove bitlbee-plugin-skype and skyped (obsolete)
   * Enable only bitlbee.service, not bitlbee.socket too
 
+  [ Sean Whitton ]
+  * Add existence check to chmod call in bitlbee-common.postinst
+    (Closes: #816200).
+
  -- dequis <dx@dxzone.com.ar>  Mon, 19 Mar 2018 22:18:43 -0300
 
 bitlbee (3.5.1-1) unstable; urgency=medium