ANDROID: cpufreq: system panic when switch interactive governor frequently

When try to suspend and resume and playback video or audio at the same time,
Power HAL will switch governor between conservative and interactive.

Before cpufreq switch to conservative governor, it will stop interactive
governor(in function: cpufreq_interactive_stop) and set "icpu->ipolicy = NULL;".
If the cpufreq_interactive_speedchange_task doesn't exit and run at the
same time, it try to get "policy = icpu->ipolicy->policy;". It will cause
system panic. Need to check pointer validatation before read.

Change-Id: I608969370738130c44b1816f073423d509a3bcd7
Signed-off-by: Zhang Bo <bo.zhang@nxp.com>
Signed-off-by: Todd Poynor <toddpoynor@google.com>