In case an AVBPrint was not complete, icecast_open() would free some
buffers that have not been allocated yet instead of freeing the data of
the AVBPrint (if they have been allocated). Because this error does not
trigger a jump to the general cleanup section any more, one can moreover
remove a (now unnecessary) initialization of a pointer.

Furthermore, finalizing an AVBPrint can fail (namely when the string
inside the AVBPrint has not been allocated yet) and so this needs to be
checked.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>