Fix divide-by-zero error in pprof. It is possible for sample contexts
to currently have no associated objects, but the cumulative statistics
are still useful, depending on how the user invokes pprof. Since
jemalloc intentionally does not filter such contexts, take care not to
divide by 0 when re-scaling for v2 heap sampling.
Install pprof as part of 'make install'.
Update pprof documentation.
