Handle read errors safely in StackLineReader to prevent buffer overreads

[jdymitarai]

Summary of Changes

1. Fix integer underflow and buffer overreads on read failures:

   • In LoadFullBuffer and LoadMore, CpuFeatures_ReadFile returns -1 on I/O error or invalid file descriptors.
   • In non-debug / release builds (NDEBUG defined), assert(read >= 0) is omitted.
   • Assigning a negative integer to unsigned size_t reader->view.size (or adding it via += read) caused reader->view.size to underflow to SIZE_MAX (18446744073709551615ULL).
   • Consequently, subsequent operations in IndexOfEol / CpuFeatures_memchr performed unbounded out-of-bounds reads past reader->buffer on the stack, leading to segmentation faults or infinite loops.

2. Defensive handling:

   • In LoadFullBuffer, if read <= 0, reset reader->view.size = 0 and return -1 (if < 0) or 0.
   • In LoadMore, if read <= 0, return immediately without modifying reader->view.size.
   • In SkipToNextLine and StackLineReader_NextLine, check read <= 0 to treat read errors as EOF cleanly.

3. Test Infrastructure & Coverage:

   • Extended test/filesystem_for_testing to support simulated read errors and FindFileOrNull(int fd) for invalid descriptors.
   • Added unit tests in test/stack_line_reader_test.cc covering:
     • Invalid file descriptor (-1) initialization and reading.
     • Mid-stream read failure after reading valid lines.
     • Read failure occurring while in line truncation skip mode.

Verification

All tests in stack_line_reader_test, string_view_test, cpuinfo_x86_test, and bit_utils_test pass cleanly on MSVC.