Unlock the mutex prior to destructing it.

[rhc54]

Thanks to Nicolas Joly for the report

[rhc54]

@hppritcha Looks like Travis is bombing over the monitoring test library (no rule to make target) which is weird - obviously unrelated, but it seems strange that only some compilers are impacted. Any ideas?

[jsquyres]

@rhc54 Remember that we have travis do "make -k", so it's building everything, even long after it hit the first compile error. The actual error is with opal/util/ethtool.c -- should be fixed by #1679. I'm waiting for some more CI completions before merging.