fix(bjs): strndup crash on scripts without directory path — likely root cause of issue #2450

[Swissola]

Fixes a crash in run_bjs_script_headless() that is the likely root cause of #2450 (crash launching app store on Cardputer ADV).

Bug

int slash = filename.lastIndexOf('/');
scriptName = strdup(filename.c_str() + slash + 1);
scriptDirpath = strndup(filename.c_str(), slash);  // ← slash can be -1

String::lastIndexOf() returns -1 when no / is present. strndup takes size_t, so -1 coerces to SIZE_MAX (~4 GB). The allocator either triggers a heap assertion panic, or returns NULL. The subsequent JS_NewString(ctx, scriptDirpath) then dereferences NULL, crashing before the script runs.

Secondary case: when slash == 0 (file at filesystem root, e.g. /main.js), strndup(p, 0) produces an empty string "" instead of "/", breaking any relative path resolution inside the script via __dirpath.

Fix

Handle the no-slash and root-slash cases before the normal strndup path:

int slash = filename.lastIndexOf('/');
if (slash < 0) {
    scriptDirpath = strdup("/");
    scriptName = strdup(filename.c_str());
} else {
    scriptName = strdup(filename.c_str() + slash + 1);
    scriptDirpath = strndup(filename.c_str(), slash == 0 ? 1 : slash);
}

Connection to #2450

The app store fetches and invokes scripts. Depending on how the invocation path is constructed, the filename passed to run_bjs_script_headless() may lack a leading /. On a memory-constrained Cardputer ADV, the near-infinite strndup allocation attempt hits the heap guard and panics, which matches the reported crash-on-launch behaviour.

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