track non-seekable stream size as usize in yyjson_read_fp

[dxbjavid]

yyjson_read_fp keeps the running byte count of a non-seekable stream in a signed long, while the buffer length itself is a usize that is overflow-checked as it grows. on builds where long is narrower than size_t (LLP64, and 32-bit targets) a stream larger than LONG_MAX wraps that counter negative, so the trailing memset writes before the buffer and yyjson_read_opts is handed a bogus length, reading and writing out of bounds. track the byte count in a usize alongside buf_size so it cannot overflow before the already-guarded buffer length does.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.55%. Comparing base (1d3f49c) to head (83d594f).

Files with missing lines	Patch %	Lines
src/yyjson.c	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #284   +/-   ##
=======================================
  Coverage   98.55%   98.55%           
=======================================
  Files           2        2           
  Lines        7726     7728    +2     
=======================================
+ Hits         7614     7616    +2     
  Misses        112      112           

Flag	Coverage Δ
unittests	98.55% <85.71%> (+<0.01%)	⬆️

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[dxbjavid]

Thank you for the review and merge. Appreciated