lib: fix a few bugs in ring buffers

lib: fix a few bugs in ring buffers

* Fix rare failure caused when end pointer is at end of buffer memory
  and a call to ringbuf_get() is made that reads all of the data in the
  buffer; start pointer was advanced past end pointer, causing some
  special handling to be skipped
* Fix ringbuf_peek() moving start pointer
* Fix use after free
* Remove extraneous assignment
* Update relevant tests

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>