MediaCodec: fix possible crash at stop & error

If an error occurred during stop() operation but the operation
completed anyway, it may lead to an assert because the error already
sent response to pending reply.

Bug: 169596940
Test: atest CtsMediaTestCases -- --module-arg CtsMediaTestCases:size:small
Test: atest mediacodecTest
Change-Id: Id88d546a22db26495a6d535cbf3e191a21ea21fd