watchdog: sbc_fitpc2_wdt, fix crash on systems without DMI_BOARD_NAME

Some systems don't provide DMI_BOARD_NAME in their DMI tables. Avoid
crash in such situations in fitpc2_wdt_init.

The fix is to check if the dmi_get_system_info return value is NULL.

The oops:
BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: [<ffffffff81253ae6>] strstr+0x26/0xa0
PGD 3966e067 PUD 39605067 PMD 0
Oops: 0000 [#1] SMP
last sysfs file: /sys/devices/system/cpu/cpu1/cache/index2/shared_cpu_map
CPU 1
Modules linked in: ...
Pid: 1748, comm: modprobe Not tainted 2.6.37-22-default #1 /Bochs
RIP: 0010:[<ffffffff81253ae6>]  [<ffffffff81253ae6>] strstr+0x26/0xa0
RSP: 0018:ffff88003ad73f18  EFLAGS: 00010206
RAX: 0000000000000000 RBX: 00000000ffffffed RCX: 00000000ffffffff
RDX: ffffffffa003f4cc RSI: ffffffffa003f4c2 RDI: 0000000000000000
...
CR2: 0000000000000000 CR3: 000000003b7ac000 CR4: 00000000000006e0
...
Process modprobe (pid: 1748, threadinfo ffff88003ad72000, task ffff88002e6365c0)
Stack: ...
Call Trace:
 [<ffffffffa004201f>] fitpc2_wdt_init+0x1f/0x13c [sbc_fitpc2_wdt]
 [<ffffffff810002da>] do_one_initcall+0x3a/0x170
...
Code: f3 c3 0f 1f 00 80 3e 00 53 48 89 f8 74 1b 48 89 f2 0f 1f 40 00 48 83 c2 01 80 3a 00 75 f7 49 89 d0 48 89 f8 49 29 f0 75 02 5b c3 <80> 3f 00 74 0e 0f 1f 44 00 00 48 83 c0 01 80 38 00 75 f7 49 89

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>