Decoder: Cleaned up parse sps function.

Postponed the initializations to decoder context
till the end of the parse sps function, after
all the error checks are done.

Bug: 37968755
Test: ran poc on ASAN-enabled build before/after
Change-Id: Ibee3383c28cede3edb68d2459565d6ce10683bbd
(cherry picked from commit 4eb72f7c935595817026b4cf4aed5ef2ff579ab5)