giochannel: Fix length_size bounds check

• giochannel: Fix length_size bounds check

  From: @heftig

  The inverted condition is an obvious error introduced by ecdf9140.

  Fixes #2323 (closed)

• io-channel test: Add coverage for g_io_channel_set_line_term(., ., -1)

  From: @smcv

• giochannel: Don't store negative line_term_len in GIOChannel struct

  From: @smcv

  Adding test coverage indicated that this was another bug in 0cc11f74.

  Fixes: 0cc11f74 "giochannel: Forbid very long line terminator strings"
  Resolves: #2323 (closed)

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This is a regression in 2.66.6 and 2.67.3.