bin/53634: mandoc dumps core building from linux

[coypu%sdf.org@localhost]

>Number:         53634
>Category:       bin
>Synopsis:       mandoc dumps core building from linux
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Sep 27 16:20:00 +0000 2018
>Originator:     coypu
>Release:        NetBSD 8.99.25
>Organization:
>Environment:
Linux lin 4.15.0-34-generic #37-Ubuntu SMP Mon Aug 27 15:21:48 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

>Description:
while building from linux, we dump core in libc man pages (getdirentries). I built with more debug info:

Core was generated by `/home/fly/obj/tooldir.Linux-4.15.0-34-generic-x86_64/bin/nbmandoc -Thtml -Oman='.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x000056057b525561 in memcpy (__len=32, __src=0x7fff13df8850, __dest=0x7c841c40) at /usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
34	  return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
(gdb) bt
#0  0x000056057b525561 in memcpy (__len=32, __src=0x7fff13df8850, __dest=0x7c841c40) at /usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
#1  mdoc_argv (mdoc=mdoc@entry=0x56057c82b9e0, line=line@entry=154, tok=tok@entry=MDOC_Bl, reta=reta@entry=0x7fff13df88f8, 
    pos=pos@entry=0x7fff13df896c, buf=buf@entry=0x56057c82bb30 ".Bl -tag")
    at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/mdoc_argv.c:367
#2  0x000056057b527106 in blk_full (mdoc=0x56057c82b9e0, tok=MDOC_Bl, line=154, ppos=1, pos=0x7fff13df896c, buf=0x56057c82bb30 ".Bl -tag")
    at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/mdoc_macro.c:1035
#3  0x000056057b5243d4 in mdoc_macro (mdoc=mdoc@entry=0x56057c82b9e0, tok=tok@entry=MDOC_Bl, line=line@entry=154, ppos=<optimized out>, 
    pos=pos@entry=0x7fff13df896c, buf=buf@entry=0x56057c82bb30 ".Bl -tag")
    at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/mdoc.c:86
#4  0x000056057b52459a in mdoc_pmacro (mdoc=mdoc@entry=0x56057c82b9e0, ln=ln@entry=154, buf=buf@entry=0x56057c82bb30 ".Bl -tag", offs=<optimized out>)
    at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/mdoc.c:403
#5  0x000056057b524a7f in mdoc_parseln (mdoc=0x56057c82b9e0, ln=154, buf=0x56057c82bb30 ".Bl -tag", offs=<optimized out>)
    at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/mdoc.c:78
#6  0x000056057b52f9b5 in mparse_buf_r (curp=curp@entry=0x56057c82b2b0, blk=..., i=<optimized out>, start=start@entry=1)
    at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/read.c:536
#7  0x000056057b52fa9b in mparse_parse_buffer (curp=curp@entry=0x56057c82b2b0, blk=..., 
    file=file@entry=0x7fff13dfaa0f "/home/fly/src/lib/libc/db/man/mpool.3")
    at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/read.c:703
#8  0x000056057b52fb44 in mparse_readfd (curp=0x56057c82b2b0, fd=<optimized out>, file=0x7fff13dfaa0f "/home/fly/src/lib/libc/db/man/mpool.3")
    at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/read.c:740
#9  0x000056057b50b7b5 in parse (curp=0x7fff13df8bd0, fd=4, file=0x7fff13dfaa0f "/home/fly/src/lib/libc/db/man/mpool.3")
    at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/main.c:828
#10 0x000056057b50c789 in main (argc=1, argv=0x7fff13df8d90) at /home/fly/src/tools/mandoc/../../external/bsd/mdocml/bin/mandoc/../../dist/main.c:545
>How-To-Repeat:
Build on Ubuntu.
>Fix: