BUG/MINOR: config: stick-table is not supported in defaults section

BUG/MINOR: config: stick-table is not supported in defaults section

Thierry discovered that the following config crashes haproxy while
parsing the config (it's probably the smallest crasher) :

   defaults
       stick-table type ip size 1M

And indeed it does because it looks for the current proxy's name which it
does not have as it's the default one. This affects all versions since 1.6.

This fix must be backported to all versions back to 1.6.