ao-tools: Use array indexing instead of addition to make gcc-10 happy

ao-tools: Use array indexing instead of addition to make gcc-10 happy

A struct with a trailing zero-length array (for variable-length data) is
treated as a zero-sized object when doing pointer arithmetic, but treated
correctly when treated as an array. This generates a warning from gcc-10

	load->data + address - load->address

while this, which is 'the same', does not:

	&load->data[address - load->address]

Signed-off-by: Keith Packard <keithp@keithp.com>