Fix OOB in SDP server continuation length

Before we read the continuation byte, we need to check
that it is still within the packet bounds.

Ignore-AOSP-First: Security fix
Test: compiles
Bug: 235102570
Tag: #security
Change-Id: Ib96293df0545e1ba848053a47202c5579e4cdccc
(cherry picked from commit cbce3382)
Merged-In: Ib96293df0545e1ba848053a47202c5579e4cdccc