vp9: fix -Wclobbered (longjmp + local variables)

vp9: fix -Wclobbered (longjmp + local variables)

Local variables used at the setjmp() site need to be marked volatile.
Relevant excerpt from the 'man longjmp':

===============
The values of automatic variables are unspecified after a call to
longjmp() if they meet all the following criteria:
·  they are local to the function that made the corresponding setjmp(3) call;
·  their values are changed between the calls to setjmp(3) and longjmp(); and
·  they are not declared as volatile.
===============

Change-Id: I093e6eeeedbf5f781d202248ca701ba2c29d3064