drivers/misc/ti-st/st_core.c: missing NULL check

Addresses https://bugzilla.kernel.org/show_bug.cgi?44431



Reported-by:  <rucsoftsec@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Pavan Savoy <pavan_savoy@ti.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>