[FIX] Unexpected behavior of get_write_interval

[IshanGrover2004]

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Closes #1606

Issue addresed

This pull request addresses issue #1606 where the get_write_interval function incorrectly returns (0,209) for a packet does not contain captions, implying that caption packets are available from index 0 to 209 which needs to be corrected.

Correction Made

The correction ensures that the correct interval (0,0) is returned, indicating that there are no captions available to write.

Implementation Details

By initialising last variable which stores last caption available by 0