Description:
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Line 1772 of streams.c is: '} else if (n == 5 && strncasecmp(path, "zlib:", 5) == 0) '

This is a bug.

If path starts with "zlib:" then n will be 4, not 5.

Since this branch can never be entered, and it only exists to support a deprecated stream type, I suggest simple deleting the entire "else" clause.

Test script:
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readfile("zlib:file.gz");

Expected result:
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gzip content is extracted, warning (Use of "zlib:" wrapper is deprecated; please use "compress.zlib://" instead) is printed

Actual result:
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Warning (failed to open stream: No such file or directory) is printed