tools/libxc: arm: Check the index before accessing the bank

tools/libxc: arm: Check the index before accessing the bank

When creating a guest with more than 3GB of memory, the 2 banks will be
used and the loop with overrunning. The code will fail later on because
Xen will deny to populate the region:

domainbuilder: detail: xc_dom_devicetree_mem: called
domainbuilder: detail: xc_dom_mem_init: mem 3096 MB, pages 0xc1800 pages, 4k each
domainbuilder: detail: xc_dom_mem_init: 0xc1800 pages
domainbuilder: detail: xc_dom_boot_mem_init: called
domainbuilder: detail: set_mode: guest xen-3.0-aarch64, address size 64
domainbuilder: detail: xc_dom_malloc            : 14384 kB
domainbuilder: detail: populate_guest_memory: populating RAM @0000000040000000-0000000100000000 (3072MB)
domainbuilder: detail: populate_one_size: populated 0x3/0x3 entries with shift 18
domainbuilder: detail: populate_guest_memory: populating RAM @0000000200000000-0000000201800000 (24MB)
domainbuilder: detail: populate_one_size: populated 0xc/0xc entries with shift 9
domainbuilder: detail: populate_guest_memory: populating RAM @0000007fad41c000-0007fb39dd42c000 (2141954816MB)
domainbuilder: detail: populate_one_size: populated 0x100/0x1e4 entries with shift 0
domainbuilder: detail: populate_guest_memory: Not enough RAM

This is because we are currently accessing the bank before checking the
validity of the index. AFAICT, on  Debian Jessie, the compiler (gcc 4.9.2) is
assuming that it's not necessary to verify the index because it's used
before. This is a valid assumption because the operand of && are
execute from from left to right.

Re-order the checks to verify the validity of the index before accessing
the bank.

The problem has been present since the introduction of the multi-bank
feature in commit 45d9867837f099e9eed4189dac5ed39d1fe2ed49 " tools: arm:
prepare domain builder for multiple banks of guest RAM".

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>