mesa: NULL check the pointer before trying to dereference it

mesa: NULL check the pointer before trying to dereference it

Duh.

Fixes issues identified by Klocwork analysis:

    Pointer 'table' returned from call to function 'calloc' at line 115
    may be NULL and will be dereferenced at line 117.

and

    Suspicious dereference of pointer 'table' before NULL check at line
    119.

NOTE: This is a candidate for the stable branches.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>