stream_wrap: remove bogus assert

Libuv may provide a NULL buffer to the uv_read callback in case of an
error, so with this assert we'd be using the api incorrectly. None of
the current DoRead implementations rely on this constraint, either.

src/stream_wrap.cc

@@ -200,7 +200,6 @@ void StreamWrap::OnReadCommon(uv_stream_t* handle,
     }
   }
 
-  assert(buf.base != NULL);
   wrap->callbacks_->DoRead(handle, nread, buf, pending);
 }