Fix erroneous getStreamTime() results in some cases.

Without this, if HAVE_GETTIMEOFDAY is defined getStreamTime() can return
a wrong value in the following case:

- start stream
- stop stream
- start stream
- call getStreamTime() immediately

In this case tickStreamTime() will not have been called since the
stream was restarted, hence lastTickTimestamp will refer to the
previous run of the stream.  getStreamTime() will therefore extrapolate
wrongly from stream._streamTime.  This patch only uses lastTickTimestamp
when it is valid.