Add error checking to 'cmd' buffer as it may not be available (ie. in the case of exec).


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@192318 91177308-0d34-0410-b5e6-96231b3b80d8
Add exec support for Linux including common support for POSIX.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@192319 91177308-0d34-0410-b5e6-96231b3b80d8