Use size_t for getLine column accumulator to prevent int overflow

[billdenney]

getLine (src/parseSyntaxErrors.h) walks src to find the column of a syntax error. The column accumulator was a signed int col and the subsequent allocation was R_Calloc(col + 1, char). On a source line wider than INT_MAX bytes, col wraps to a negative int; the allocation either fails or is undersized, and the following memcpy(buf, src + i, col) writes past the buffer.

Switch the accumulator to size_t, add an explicit col == INT_MAX guard inside the column-counting loop, and add an i + col > INT_MAX overflow check before the cast back to int for the R_Calloc call.

Adds tests/testthat/test-mem-getline-col-overflow.R as a regression test. The boundary test is skip()ed because it requires constructing an INT_MAX-byte single-line input.

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.44%. Comparing base (ca1ffea) to head (849b320).

Files with missing lines	Patch %	Lines
src/parseSyntaxErrors.h	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   89.44%   89.44%   -0.01%     
==========================================
  Files          60       60              
  Lines        6360     6365       +5     
==========================================
+ Hits         5689     5693       +4     
- Misses        671      672       +1     

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