Coverity: Don't return freed pointers from command* along error path.

Coverity: Don't return freed pointers from command* along error path.

If the external command failed to run, we could free up the allocated
*stdoutput and *stderror pointers, but then return those freed
pointers to the caller.  The caller usually tries to print and free
*stderror, so this is a serious error.

Instead, return *stdoutput as NULL, and *stderror pointing to a
generic error message.