cpupower: Fix comparing pointer to 0 coccicheck warns

Fix cocciccheck wanrns found by:
make coccicheck MODE=report M=tools/power/cpupower/

tools/power/cpupower/utils/helpers/bitmask.c:29:12-13: WARNING comparing pointer to 0, suggest !E
tools/power/cpupower/utils/helpers/bitmask.c:29:12-13: WARNING comparing pointer to 0
tools/power/cpupower/utils/helpers/bitmask.c:43:12-13: WARNING comparing pointer to 0

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>