run_thread_for_a_while: Make the computation of done_this_time less
bogus, and in particular ensure that it can't be zero if in fact the
thread did do some useful work.  Fix up a couple of associated
assertions.  Fixes #336435.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14386 a5019735-40e9-0310-863c-91ae7b9d1cf9
Update.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14387 a5019735-40e9-0310-863c-91ae7b9d1cf9