# Fix — file I/O NULL-handle crash (`Showcase11` SIGSEGV)

## Symptom

`Showcase11` appeared to segfault "when entering an integer". The
crash was actually in the `Files` section that runs just after: the
program writes/reads `gen_ssa/_showcase11.txt` **relative to the
current directory**. Run from anywhere lacking `gen_ssa/` (repo root,
or from inside `gen_ssa/`), `Files.OpenWrite` returns Modula-2 `NIL`
(a C `FILE*` of `0`), and the shim then called `fputc(NULL)` /
`getc(NULL)` → SIGSEGV (exit 139).

## Fix

`runtime/syslib/shim.c`: every file routine now treats handle `0` as a
safe no-op / returns `0` — `m2fgetc`, `m2fputc`, `m2fputs`,
`m2freadint`, `m2fwriteint`, `m2fwriteln`, `m2fread`,
`m2fwriteintw` (`m2fclose` was already guarded). A failed open now
degrades gracefully instead of crashing.

## Result

- Run from `compiler/`: `Showcase11` unchanged (exit **80**).
- Run elsewhere: no crash (prints `0` for the unreadable file, exit 38).
- New regression test `compiler/tests/t_badfile.mod`: opens a path
  that cannot exist, then does `WriteChar`/`WriteInt`/`ReadInt`/
  `Close` on the NIL handle; expects exit 42. Registered as
  `TBadFile` in `run_tests.sh`.

Suite **107/107**.

## Files

`runtime/syslib/shim.c`, `compiler/tests/t_badfile.mod`,
`compiler/run_tests.sh`.