Stream: avoid potential infinite loop at preread phase.

Stream: avoid potential infinite loop at preread phase.

Previously the preread phase code ignored NGX_AGAIN value returned from
c->recv() and relied only on c->read->ready.  But this flag is not reliable and
should only be checked for optimization purposes.  For example, when using
SSL, c->read->ready may be set when no input is available.  This can lead to
calling preread handler infinitely in a loop.