Running tar.pipeToFileSystem compressed_mingw_includes.tar file from #24732
finishes in infinite loop calling defaultReadVec with:
r.seek = 1024
r.end = 1024
r.buffer.len = 1024
first.len = 512
that combination calls vtable.stream with 0 capacity writer and loops
forever.

Comment is to use whichever has larger capacity, and this fix reflects that.