Added boundaries checks into frame parser.

Added boundaries checks into frame parser.

The ngx_quic_parse_frame() functions now has new 'pkt' argument: the packet
header of a currently processed frame.  This allows to log errors/debug
closer to reasons and perform additional checks regarding possible frame
types.  The handler only performs processing of good frames.

A number of functions like read_uint32(), parse_int[_multi] probably should
be implemented as a macro, but currently it is better to have them as
functions for simpler debugging.