http_recv: Fix reading of the HTTP OK message pattern.

http_recv: Fix reading of the HTTP OK message pattern.

This bug leads to sound artefacts at the beginning of the
stream caused by calling recv_pattern() with a buffer size
larger than the HTTP OK message.

It may thus happen that the buffer used by recv_pattern() contains
not only the HTTP OK message but also the beginning of the stream.
Since that buffer is discarded by recv_pattern(), the first few
bytes of the stream are lost if this happens.

Fix it by passing the exact length of the HTTP OK message to
recv_pattern().