Hi Jakub,
with the way that chunk_size < 1 is handled for gomp_iter_dynamic_next:

(1) chunk_size <= -1: wraps into large unsigned value, seems to work though.
(2) chunk_size == 0:  infinite loop

The (2) behavior is obviously not desired. This patch fixes this by changing
the chunk_size initialization in gomp_loop_init to "max(1,chunk_size)"

The OMP_SCHEDULE parsing in libgomp/env.c has also been adjusted to reject
negative values.

Tested without regressions, and a new testcase for the infinite loop behavior added.
Okay for trunk?

Thanks,
Chung-Lin

libgomp/ChangeLog:
	* env.c (parse_schedule): Make negative values invalid for chunk_size.
	* loop.c (gomp_loop_init): For non-STATIC schedule and chunk_size <= 0,
	set initialized chunk_size to 1.

	* testsuite/libgomp.c/loop-28.c: New test.