backtrace_frame should handle assignment to element zero for compatibility
Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1799
Reported by Martin Stjernholm mast@roxen.com
In e.g. Roxen there's code that does this:
foreach (reverse (err[1]), mixed ent) {
string file, func, descr;
int line;
if (arrayp (ent)) {
if (sizeof (ent) && stringp (ent[0]))
if (ent[0][..sizeof (cwd) - 1] == cwd)
file = ent[0] = ent[0][sizeof (cwd)..];