drivers/base/memory.c: fix kw reported issue

Fix KW reported 'NULL pointer dereference' issue. This is fixed in the
upstream as part of 848e19ad3c335 ("drivers/base/memory.c: drop the
mem_sysfs_mutex") but the reason not to pick this up as it is that it
contains the additional changes not so important for 4.19 kernels.

Change-Id: I7cb5b5202c29b280c853b849f29201d70520faf5
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>