rtcpbuffer: fix function guards with side effects
Code in g_return_*() must not have side effects, as it might be compiled out if -DG_DISABLE_CHECKS is used, in which case we would read garbage off the stack.
Code in g_return_*() must not have side effects, as it might be compiled out if -DG_DISABLE_CHECKS is used, in which case we would read garbage off the stack.