i965: quieten compiler warning about out-of-bounds access

i965: quieten compiler warning about out-of-bounds access

gcc 4.9.3 shows the following error:

brw_vue_map.c:260:20: warning: array subscript is above array bounds
[-Warray-bounds]
    return brw_names[slot - VARYING_SLOT_MAX];

This is because BRW_VARYING_SLOT_COUNT is a valid value for the enum
type. Adding an assert will generate no additional code but will teach
the compiler to not complain.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>