libdvbv5: fix extension descriptor handler

libdvbv5: fix extension descriptor handler

There are several issues at the extension handler:

1) the size is wrong, as it is getting data from
   descriptors.c, instead of from the extensions;

2) there are memory leaks;

3) It doesn't print the extension descriptors.

Fix them.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>