crash with genders on macOS 10.13

[wadey]

This issue was mentioned at the end of #95:

There is one remaining problem, however. If I add genders to the Travis build for OSX then crashes start again whenever the genders module is used. I was not successful debugging this iteratively via Travis, so if anyone gets a good backtrace, that would help.

If I build and run pdsh with the following parameters I get the crash:

$ git show -1 --oneline | head
00f66b2 Merge pull request #106 from grondo/travis-fixes

$ ./configure --with-ssh --without-rsh --with-nodeupdown --without-xcpu --with-genders --without-dshgroups --enable-debug
$ make

$ ./src/pdsh/pdsh -g all
pdsh(48415,0x7fffb7021340) malloc: *** error for object 0x7fc173d01060: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug

Unfortunately, gdb is also broken in macOS 10.13 which makes this much harder to debug: https://sourceware.org/bugzilla/show_bug.cgi?id=20981 . The only info I can get from gdb is:

(gdb) run -g all
Starting program: /Users/wade/src/pdsh/src/pdsh/pdsh -g all
[New Thread 0x1c03 of process 48558]
warning: unhandled dyld version (15)
pdsh(48558,0x7fffb7021340) malloc: *** error for object 0x100400140: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug

Thread 2 received signal SIGABRT, Aborted.
0x00007fff7daa3fce in ?? ()
(gdb) bt
#0  0x00007fff7daa3fce in ?? ()
#1  0x00007fff7dbe1150 in ?? ()
#2  0x00007fffb7021340 in ?? ()
#3  0x00007ffeefbfea98 in ?? ()
#4  0x0000000000000000 in ?? ()