egl/dri: Avoid out of bounds array access

indexConfigAttrib iterates over every index in the dri driver, possibly
exceeding __DRI_ATTRIB_MAX. In other words, if the dri driver has newer
attributes libEGL will end up reading from uninitialized memory through
dri2_to_egl_attribute_map[].

Signed-off-by: Kevin Strasser <kevin.strasser@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 70b36c0e)