rtpgsmpay: fix accidental garbage data before actual payload

rtpgsmpay: fix accidental garbage data before actual payload

Do not allocate payload size outbuf if appending payload buffer.

The commit 137672ff1824948bda4b1b1967de8c24a0055b67 attached payload
to the output buffer but forgot to remove payload allocation.  That
effectively doubled payload size and add zero'ed or random bytes.

Makes the following pipeline work again:

gst-launch-1.0 -v audiotestsrc wave=2 ! gsmenc ! rtpgsmpay ! rtpgsmdepay ! gsmdec ! autoaudiosink

https://bugzilla.gnome.org/show_bug.cgi?id=784616