Initialise VPS buffer to NULL in constructor

Missing initialisation of this pointer could lead to an incorrect free
if the ARTWriter object is cleared immeddiately after the constructor
call.

Bug: 287298721
Test: rtp_writer_fuzzer
(cherry picked from https://partner-android-review.googlesource.com/q/commit:2710696b001f2e95586151c1ee337a4e3c4da48a)
Change-Id: I08eacd7a0201bc9a41b821e20cae916d8870147a