aacenc: Adjust the initial offset for PNS values

This commit adjusts the intial offset for PNS values, introduced
with commit f7f71b57 earlier. This
commit shifts the value in such a way that no further offsets are
required in the aaccoder.c file. Earlier version of the PNS patch had 2 offsets in both the aaccoder and aacenc.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>