core: ofnode: Fix ASAN-reported stack-buffer-overflow in of_get_address

v2019.04-rc3 sandbox U-Boot fails to boot when compiled with
 -fsanitize=address and linked against -lasan, reporting [1].

Git bisecting shows that the issue is contributed by v2019.01 commit
1678754f

 ("core: ofnode: Fix ofnode_get_addr_index function").

The root cause seems to be the mismatch between sizeof(u64) and
sizeof(fdt_size_t) on sandbox. Luckily, thanks to the fact that the
size argument of both of_get_address() and fdtdec_get_addr_size_fixed()
is optional, we can pass NULL in its place, avoiding the problem.

[1] Backtrace reported by ASAN (gcc 8.1.0):

$> ./u-boot -d arch/sandbox/dts/sandbox.dtb
[..]
Reviewed-by: Simon Glass <sjg@chromium.org>

=================================================================
==10998==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffcc2331140 at pc 0x0000004eeeb0 bp 0x7ffcc2330f80 sp 0x7ffcc2330f70
WRITE of size 8 at 0x7ffcc2331140 thread T0
    #0 0x4eeeaf in of_get_address drivers/core/of_addr.c:154
    #1 0x4f7441 in ofnode_get_addr_index drivers/core/ofnode.c:263
    #2 0x5b2a78 in sb_eth_ofdata_to_platdata drivers/net/sandbox.c:422
    #3 0x4dccd8 in device_probe drivers/core/device.c:407
    #4 0x753170 in eth_initialize net/eth-uclass.c:428
    #5 0x47d9bf in initr_net common/board_r.c:557
    #6 0x6bcfa7 in initcall_run_list lib/initcall.c:30
    #7 0x47e1fe in board_init_r common/board_r.c:859
    #8 0x4060e5 in main arch/sandbox/cpu/start.c:356
    #9 0x7fb8d135482f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #10 0x40a3a8 in _start (/path/to/u-boot/u-boot+0x40a3a8)

Address 0x7ffcc2331140 is located in stack of thread T0 at offset 32 in frame
    #0 0x4f72b8 in ofnode_get_addr_index drivers/core/ofnode.c:255

  This frame has 3 object(s):
    [32, 36) 'size' <== Memory access at offset 32 partially overflows this variable
    [96, 100) 'flags'
    [160, 168) 'node'
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow drivers/core/of_addr.c:154 in of_get_address
Shadow bytes around the buggy address:
  0x10001845e1d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10001845e1e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10001845e1f0: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
  0x10001845e200: 04 f2 f2 f2 f2 f2 f2 f2 04 f2 f2 f2 f2 f2 f2 f2
  0x10001845e210: 04 f2 f2 f2 f3 f3 f3 f3 00 00 00 00 00 00 00 00
=>0x10001845e220: 00 00 00 00 f1 f1 f1 f1[04]f2 f2 f2 f2 f2 f2 f2
  0x10001845e230: 04 f2 f2 f2 f2 f2 f2 f2 00 f2 f2 f2 f3 f3 f3 f3
  0x10001845e240: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10001845e250: 00 00 00 00 f1 f1 f1 f1 00 00 f2 f2 f3 f3 f3 f3
  0x10001845e260: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1
  0x10001845e270: f1 f1 00 f2 f2 f2 f3 f3 f3 f3 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==10998==ABORTING

'To' list:
 git log --since=1year drivers/core/ofnode.c | grep "\-by: .*@" | \
     sed 's/.*-by: //' | sort | uniq -c | sort -rn
     10 Simon Glass <sjg@chromium.org>
      3 Mario Six <mario.six@gdsys.cc>
      2 Martin Fuzzey <mfuzzey@parkeon.com>
      2 Marek Vasut <marek.vasut+renesas@gmail.com>
      1 Tom Rini <trini@konsulko.com>
      1 Masahiro Yamada <yamada.masahiro@socionext.com>
      1 Keerthy <j-keerthy@ti.com>
      1 Jens Wiklander <jens.wiklander@linaro.org>
      1 Bin Meng <bmeng.cn@gmail.com>

Fixes: 1678754f

 ("core: ofnode: Fix ofnode_get_addr_index function")
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>