Handle AssertionError when mention exceeds max_seq_length

[AVBelyy]

If mention_tokens length exceeds max_seq_length - 4, this forces left_quota and right_quota to become 0 or negative, which leads to undesired behavior and ultimately an AssertionError at the end of the function. This commit fixes that.

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!