Fixing warnings shouldn't introduce a crasher.

Fix the warning the correct way without making things crash when ENABLE_MUTEX_ERROR_CHECKING is non enabled.

<rdar://problem/17703039>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@213394 91177308-0d34-0410-b5e6-96231b3b80d8