thread.c (set_trace_func, thread_set_trace_func_m): reset tracing
state when set_trace_func hook is removed. This is workaround patch
to force to reset tracing state that is broken by continuation call.
a patch from James M. Lawrence. [Feature #4347] [ruby-core:34998]
test/ruby/test_continuation.rb (class TestContinuation): add a test
for above. a patch from James M. Lawrence.