rtcpbuffer: test for len instead of type

The function rtcp_packet_min_length() returns a length for each known type
and -1 for unknown types. This change fixes the test accordingly and silences
the following warning.

gstrtcpbuffer.c:567:12: error: comparison of constant -1 with expression of type 'GstRTCPType' is always false
      [-Werror,-Wtautological-constant-out-of-range-compare]
  if (type == -1)
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