assertion "!"reached"" failed in pp_xrentry

Seen in specimen sri/lzw-add-with-overflow of the instigator corpus.

NB: Parse succeeds - this occurs during pretty-printing of the result AST.

Backtrace:

#0  thrkill () at /tmp/-:3
#1  0x21d49e6228681cdd in ?? ()
#2  0x0000039d3c9c92ae in _libc_abort () at /usr/src/lib/libc/stdlib/abort.c:51
#3  0x0000039d3c9e3712 in _libc___assert2 (file=Variable "file" is not available.
)
    at /usr/src/lib/libc/gen/assert.c:52
#4  0x0000039b34ce6f11 in pp_xrentry (stream=0x39d3ca6faa8, tok=0x39e0bf6cac8, 
    indent=22, delta=2) at pdf.c:452
#5  0x0000039d82473140 in h_pprint (stream=0x39d3ca6faa8, tok=0x39e0bf6cac8, 
    indent=20, delta=2) at build/debug/src/pprint.c:95
#6  0x0000039d82472ffa in h_pprint (stream=0x39d3ca6faa8, tok=0x39d39de7f18, 
    indent=18, delta=2) at build/debug/src/pprint.c:79
#7  0x0000039d82472ffa in h_pprint (stream=0x39d3ca6faa8, tok=0x39d39de7f98, 
    indent=16, delta=2) at build/debug/src/pprint.c:79
#8  0x0000039d82472ffa in h_pprint (stream=0x39d3ca6faa8, tok=0x39df84fa018, 
    indent=14, delta=2) at build/debug/src/pprint.c:79
#9  0x0000039b34ce707d in pp_parseresult (stream=0x39d3ca6faa8, 
    tok=0x39db8807498, indent=14, delta=2) at pdf.c:472
#10 0x0000039d82473140 in h_pprint (stream=0x39d3ca6faa8, tok=0x39db8807498, 
    indent=12, delta=2) at build/debug/src/pprint.c:95
#11 0x0000039d82472ffa in h_pprint (stream=0x39d3ca6faa8, tok=0x39db8807518, 
    indent=10, delta=2) at build/debug/src/pprint.c:79
#12 0x0000039d82472ffa in h_pprint (stream=0x39d3ca6faa8, tok=0x39d3d00bf70, 
    indent=8, delta=2) at build/debug/src/pprint.c:79
#13 0x0000039d82472ffa in h_pprint (stream=0x39d3ca6faa8, tok=0x39d7be679b0, 
    indent=6, delta=2) at build/debug/src/pprint.c:79
#14 0x0000039d82472ffa in h_pprint (stream=0x39d3ca6faa8, tok=0x39df5cadcf0, 
    indent=4, delta=2) at build/debug/src/pprint.c:79
#15 0x0000039d82472ffa in h_pprint (stream=0x39d3ca6faa8, tok=0x39e0cec1130, 
    indent=2, delta=2) at build/debug/src/pprint.c:79
#16 0x0000039d82472ffa in h_pprint (stream=0x39d3ca6faa8, tok=0x39e0cec1b68, 
    indent=0, delta=2) at build/debug/src/pprint.c:79
#17 0x0000039d82473189 in h_pprintln (stream=0x39d3ca6faa8, tok=0x39e0cec1b68)
    at build/debug/src/pprint.c:103
#18 0x0000039b34cfa717 in main (argc=1, argv=0x7f7ffffd7840) at pdf.c:5167

Context:

#4  0x0000039b34ce6f11 in pp_xrentry (stream=0x39d3ca6faa8, tok=0x39e0bf6cac8, 
    indent=22, delta=2) at pdf.c:452
452                     assert(!"reached");
Current language:  auto; currently minimal
(gdb) list
447             case XR_OBJSTM:
448                     fprintf(stream, "{ \"type\":\"objstm\", \"stm\":%zu, "
449                         "\"idx\":%zu }", xr->o.stm, xr->o.idx);
450                     break;
451             default:
452                     assert(!"reached");
453             }
454     }
455     
456     void
(gdb) list pp_xrentry
430             return NULL;
431     }
432     
433     void
434     pp_xrentry(FILE *stream, const HParsedToken *tok, int indent, int delta)
435     {
436             XREntry *xr = H_CAST(XREntry, tok);
437     
438             switch (xr->type) {
439             case XR_FREE:
(gdb) print xr->type
$1 = 91