Previously it was possible to cause a 64bit signed to 32bit
unsigned overflow. `from_stretcher` is pframes_t aka. uint32_t.
With int64_t arguments a std::min() expression producing negative
result will result in large 32bit values:
(pframes_t) std::min<int64_t>(1024, 176400 - 187392) = 4294956304
This produced a segfault when used as n_samples to copy in
buf.accumulate_from()
5658e18815