stream: Fix stall in Transform under very specific conditions

The stall is exposed in the test, though the test itself asserts before
it stalls.

The test is constructed to replicate the stalling state of a complex
Passthrough usecase since I was not able to reliable trigger the stall.

Some of the preconditions for triggering the stall are:
  * rs.length >= rs.highWaterMark
  * !rs.needReadable
  * _transform() handler that can return empty transforms
  * multiple sync write() calls

Combined this can trigger a case where rs.reading is not cleared when
further progress requires this. The fix is to always clear rs.reading.