intel/vec4: fix out of bounds read

NIR_MAX_VEC_COMPONENTS was bumped from 4 to 16 in a8ec4082
(2019.03.09, merged 2019.12.21)

float[4] array was added in acd7796a
(2019.06.11, merged 2019.07.11)

Found by Coverity.
Closes: #3014

Signed-off-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Fixes: a8ec4082 ("nir+vtn: vec8+vec16 support")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <!6067>
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