Memcached: stricten header validation.

Memcached: stricten header validation.

An invalid memcached reply that started with '\n' could cause
segmentation fault.

An invalid memcached reply "VALUE / 0 2\r?ok\r\nEND\r\n" was
considered as a valid response.

In addition, if memcached reports that the key was not found,
set u->headers_in.content_length_n to 0.  This ensures that
ngx_http_memcached_filter() will not be called while previous
code relied on always intercepting 404.  Initialization of
ctx->rest was moved to where it belongs.