cpupower: Fix NULL but dereferenced coccicheck errors

Fix NULL but dereferenced coccicheck errors found by:

make coccicheck MODE=report M=tools/power/cpupower

tools/power/cpupower/lib/cpufreq.c:384:19-23: ERROR: first is NULL but dereferenced.
tools/power/cpupower/lib/cpufreq.c:440:19-23: ERROR: first is NULL but dereferenced.
tools/power/cpupower/lib/cpufreq.c:308:19-23: ERROR: first is NULL but dereferenced.
tools/power/cpupower/lib/cpufreq.c:753:19-23: ERROR: first is NULL but dereferenced.

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