Fix invalid free of static longjmp scratch buffer

[aspeer]

I have forked the zeroperl repo to support a WASM build of my WebDyne framework. During that build I saw repeated traps in the Cloudflare WASM provider which were traced to an invalid free.

This PR seeks to fix that issue.

When a captured setjmp resumes after longjmp, _asyncjmp_setjmp_internal
passes longjmp_buf_ptr to free(). _asyncjmp_longjmp points that field at
its static tmp_longjmp_buf, so recovery attempts to free non-heap storage.
Clear the pointer instead. The capture buffer size and Asyncify ABI are unchanged.

Adds a native AddressSanitizer regression that includes the production state
machine and exercises capture followed by 100 longjmp recovery cycles.

Validation on macOS arm64 with Apple clang version 21.0.0 (clang-2100.1.1.101):

• Untouched upstream: AddressSanitizer reports a bad free of
  _asyncjmp_longjmp.tmp_longjmp_buf on the first recovery.
• Patched: all 100 recovery cycles pass under AddressSanitizer.
• git diff --check passes.