The error is stating that the file /etc/malloc.conf is not found.

13542 znc      NAMI  "/etc/malloc.conf"
 13542 znc      RET   readlink -1 errno 2 No such file or directory

I'm not sure how FreeBSD's malloc works, but this seems to be a configuration issue with your operating system and not a problem in ZNC. Please refer to the man page for malloc.conf

Also, that's not a core dump and there are no write() calls in there (why do you think it is whining about malloc?).

If you want to report a crash, grab gdb and get a backtrace (gdb znc, run, bt, bt full)

http://wiki.znc.in/Debugging

it's whining about malloc because on freebsd /etc/malloc.conf does not exist and that's a kdump the freebsd equivalent to gdb I can see the errors without the need of gdb.

If you have an error message then it would be nice to tell us about that error message. Believe it or not, error messages could actually contain useful information.

Also, no. kdump is not the freebsd equivalent to gdb. The freebsd equivalent to gdb is gdb. The freebsd equivalent to strace is kdump.

[root@kurobachan /usr/local/bin]# gdb ./znc
GNU gdb 6.1.1 [FreeBSD]
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i386-marcel-freebsd"...(no debugging symbols found)...
(gdb) handle SIGPIPE nostop
Signal Stop Print Pass to program Description
SIGPIPE No Yes Yes Broken pipe
(gdb) run -D
Starting program: /usr/local/bin/znc -D
(no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...Error while reading shared library symbols:
Dwarf Error: wrong version in compilation unit header (is 4, should be 2) [in module /usr/local/lib/gcc5/libstdc++.so.6](no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...[New LWP 100130](no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...[New Thread 2a403080 (LWP 100130/znc)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 2a403080 (LWP 100130/znc)]
0x00000000 in ?? ()
(gdb)

Did I point you at http://wiki.znc.in/Debugging already? Please read it. You didn't provide neither debug symbols to gdb, nor stack trace to us.

[root@kurobachan /usr/local/bin]# gdb ./znc
GNU gdb 6.1.1 [FreeBSD]
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i386-marcel-freebsd"...Dwarf Error: wrong version in compilation unit header (is 4, should be 2) [in module /usr/local/bin/znc]

(gdb) handle SIGPIPE nostop
Signal Stop Print Pass to program Description
SIGPIPE No Yes Yes Broken pipe
(gdb) run -D
Starting program: /usr/local/bin/znc -D
(no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...Error while reading shared library symbols:
Dwarf Error: wrong version in compilation unit header (is 4, should be 2) [in module /usr/local/lib/libstdc++.so.6](no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...[New LWP 100079](no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...(no debugging symbols found)...[New Thread 2a403080 (LWP 100079/znc)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 2a403080 (LWP 100079/znc)]
0x00000000 in ?? ()
(gdb) bt full
#0 0x00000000 in ?? ()
No symbol table info available.
#1 0x29c01038 in ?? () from /usr/local/lib/libstdc++.so.6
No symbol table info available.
#2 0x29cd0220 in ?? () from /usr/local/lib/libstdc++.so.6
No symbol table info available.
#3 0x00000000 in ?? ()
No symbol table info available.
(gdb)

It's still compiled without debug symbols...

GNU gdb 6.1.1 [FreeBSD] is too old, you can install new version gdb V8.1 to solve this problem

pkg install gdb

and add /usr/local/bin/gdb to your PATH environment