In VuklanManager make sure we have a valid semaphore context before destroying it.

If errors occured during finishFrame that caused us to not have a
semaphore or possibly destroy the semaphore early we will end up
with a null mDestroySemaphoreContext in swapBuffers which we tried
to destroy unconditionally.

I haven't repro'd the connected bug, but based on the crash stack
this seems like the likely cause.

Test: manual code inspection and build.
Bug: 191950033
Change-Id: I0fbd33edff3552b91b980da9e3b2c45bc52a2dd0