I encountered the following situation with xelatex.
$ echo x | xelatex \\showthe\\everyjob
This is XeTeX, Version 3.14159265-2.6-0.99996 (TeX Live 2016)
It seems that in the given example,
s = 0 and len < 0 in the following:
/* texmfmp.c */
2815: string
2816: gettexstring (strnumber s)
2817: {
2818: unsigned bytesToWrite = 0;
2819: poolpointer len, i, j;
2820: string name;
2821: len = strstart[s + 1 - 65536L] - strstart[s - 65536L];
2822: name = xmalloc(len * 3 + 1); /* max UTF16->UTF8 expansion
2823: (code units, not bytes) */
Best,
Akira
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