Router: checking a buffer before accessing its memory fields.
Router: checking a buffer before accessing its memory fields.

This fixes the router's crash on buildbot; the reason was an unexpected 'last'
response from the application to the router arriving before the response
headers.  The last buffer is not a memory buffer, so the result of accessing
memory fields is unpredictable.

The unexpected 'last' message was caused by an error in libunit; fixed in
fee8fd855a00.

nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit