Fix improper assignment in aac_enc_fuzzer

This commit updates the fuzzer to use correct value of
kSignalingModesSize and prevent OOB access on kSignalingModes

Test: ./aac_enc_fuzzer
Bug: 180220144

Change-Id: Ic9d01c5c166f2291d8d8ffacaa7813fb1560322b
1 file changed