fix: rust bitstream segfault

[hrideshmg]

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---

This PR fixes a bug where an "exhausted" C bitstream (one that has been fully read, where pos == end) would lose its buffer context when converted to Rust and back. The current logic treats this state the same as an uninitialized stream causing the original buffer location to be lost.

The fix ensures that even an exhausted stream creates a zero-length Rust slice that retains the original base pointer so that the pointers are preserved during the conversions.

[ccextractor-bot]

CCExtractor CI platform finished running the test files on windows. Below is a summary of the test results, when compared to test for commit 6300bb7...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	4/7
DVD	3/3
DVR-MS	2/2
General	27/27
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	86/86
Teletext	21/21
WTV	11/13
XDS	34/34

NOTE: The following tests have been failing on the master branch as well as the PR:

• ccextractor --stdout --quiet --no-fontcolor 79a51f3500..., Last passed:

  Never

• ccextractor --stdout --quiet --no-fontcolor 767b546f96..., Last passed:

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• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2..., Last passed:

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  Never

• ccextractor --out=srt --latin1 df3b4d62d3..., Last passed:

  Never

Congratulations: Merging this PR would fix the following tests:

• ccextractor --service 1 --out=txt f17524b53f..., Last passed: Never
• ccextractor --autoprogram --out=srt --latin1 f1422b8bfe..., Last passed: Never
• ccextractor --datapid 5603 --autoprogram --out=srt --latin1 --teletext 85c7fc1ad7..., Last passed: Never
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• ccextractor --autoprogram --out=ttxt --latin1 --ucla --xds 0069dffd21..., Last passed: Never

All tests passing on the master branch were passed completely.

Check the result page for more info.

[ccextractor-bot]

CCExtractor CI platform finished running the test files on linux. Below is a summary of the test results, when compared to test for commit 39e051b...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	7/7
DVD	3/3
DVR-MS	2/2
General	27/27
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	85/86
Teletext	21/21
WTV	13/13
XDS	34/34

NOTE: The following tests have been failing on the master branch as well as the PR:

• ccextractor --unicode c83f765c66..., Last passed:

  Never

Congratulations: Merging this PR would fix the following tests:

• ccextractor --service 1 --out=txt f17524b53f..., Last passed: Never

All tests passing on the master branch were passed completely.

Check the result page for more info.

[prateekmedia]

LGTM!