Buffer for application prefork request allocated from temp conf mem_pool.
If error response from main process received before buffer completion handler,
temp conf mem_pool destroyed and router may crash in completion handler.

Assertion "src/nxt_buf.c:208 assertion failed: data == b->parent" triggered
when NXT_DEBUG_ALLOC enabled in configure.

This patch disables completion handler and memory allocated for buffer
released with memory pool.

Previously, an error during the prefork phase triggered assert:

 src/nxt_port.c:27 assertion failed: port->pair[0] == -1

and resulted in exiting of the main process.

This could be easily reproduced by pushing a configuration with "rootfs",
when daemon is running without required permissions.

Found by Coverity (CID 361277).

This closes #441 PR on GitHub.