Initialization bug on some OpenCL backends

Some OpenCL backends fail randomly on the Taylor-Green example:

== Iteration :   0, from t =    0.0 to t = 0.00001 ==
>Parameter |υ|₊ set to [11889326670805569262372616780801638400.00   1.00   0.00].
>Parameter dt_cfl set to 4.12869345519e-39.
>Parameter |ω|₊ set to [  1.00   1.00   2.00].
>Parameter dt_lcfl set to 0.0624998807909.
>Parameter CFL* set to 0.5.
>Parameter dt set to 4.12869345519e-39.

This has been confirmed on the following platforms:

• POCL (CPU backend using pthread)
• Intel OpenCL 2.0 GPU Driver Package

To reproduce this bug, just run the Taylor-Green example untill the bug appears:

python examples/taylor_green/taylor_green.py -VC -maxit 2 --opencl-platform-id=...

Other examples might trigger this bug.