Staging: most: mostcore/core.c. Fix "Using plain integer as NULL pointer" warnings

This patch fixes the warning generated by sparse: "Using plain integer
as NULL pointer" by replacing the pointer test against 0 with a logical test.

Signed-off-by: Adrian Remonda <adrianremonda@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>