clover: Don't crash on NULL global buffer objects.

clover: Don't crash on NULL global buffer objects.

Specs say "If the argument is a buffer object, the arg_value
pointer can be NULL or point to a NULL value in which case a NULL
value will be used as the value for the argument declared as a
pointer to __global or __constant memory in the kernel."

So don't crash when somebody does that.

v2: Insert NULL into input buffer instead of buffer handle pair
    Fix constant_argument too
    Drop r600 driver changes

v3: Fix inserting NULL pointer

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>