In messages/fmt.c, extend the %s conversion to allow a NULL argument to print as "(null)".  I don't know if this is standard, but it's what glibc does.
In messages/fmt.c, extend the %s conversion to allow a NULL argument
to print as "(null)".  I don't know if this is standard, but it's what
glibc does.

On ranger, some MPI libraries call dlopen(NULL, ...) and this was
causing hpcrun to segfault with '-dd ALL' turned on.

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