Avoid cross product with zero vector in rotational pulling.

Fixes #1431 (rotation/flex-t regression test failing on BG/Q)

In do_flex_lowlevel() we checked (by mistake!) for xj-xcn being
zero, although we need to check for yj0-ycn being zero, since
we use yj0-ycn in a cross product in the following lines of code.
I now also replaced the direct check (0 == norm(...)) by checking
what gmx_numzero(norm(...)) returns. The latter replacement
was also applied in the do_flex2_lowlevel() routine. Note that there
the check for small xj-xcn was and is actually correct.

Change-Id: I972b6d67a81e30f297db286cd2224f66753a20aa