S_hfreeentries: collapse two loops

S_hfreeentries: collapse two loops

S_hfreeentries() has two nested infinite loops: the inner one
does one sweep through all buckets, freeing all entries in each bucket;
the outer loop repeats the process if new keys have been added in the
meantime.
Collapse these two into a single 'while (keys) {}' loop.
Should be functionally the same, but simpler.