Fix OSS-Fuzz fuzzer build: memcheck/expr_exit defaults and VM cleanup

[xingyaner]

The OSS-Fuzz build invokes ./configure -Z, which previously aborted with Can only enable OSSFUZZ when MEMCHECK is enabled. This change makes -Z automatically enable memcheck instead of erroring, because memcheck mode is what makes bc_vm_shutdown() run the full heap cleanup. Without memcheck, the built fuzzers crashed at runtime with Assertion '!BC_ENABLE_OSSFUZZ || BC_EXPR_EXIT == 0' failed (src/vm.c:1481), since the default bc.expr_exit/dc.expr_exit were 1; the fuzz/OSS-Fuzz modes now set both defaults to 0 to satisfy that assertion. LeakSanitizer additionally reported a Direct leak of 6400 byte(s) originating from bc_program_exec, because LLVMFuzzerTestOneInput freed only bc_fuzzer_data and never released the VM state allocated by bc_main()/dc_main(). The fuzzers now call bc_vm_atexit(s) before returning, which under memcheck frees all VM heap state and eliminates the leaks. With these changes, build_fuzzers and check_build pass for both the AFL and libFuzzer engines (address sanitizer, x86_64), and a normal non-fuzz build shows no regression.

[gavinhoward]

I'm so glad you found the leak problem!

Only one note: on the memcheck check in configure.sh, you need to remove the line with usage because usage prints an error message and exits with an error. So memcheck=1 does nothing, and the rest of the script will not run.

Other than that, because you did the legwork, it looks great, and I'll approve it.

[xingyaner]

Thanks for catching this. I've updated the code accordingly.