tools/power x86_energy_perf_policy: Fix potential NULL pointer dereference

tools/power x86_energy_perf_policy: Fix potential NULL pointer dereference

In err_on_hypervisor(), strstr() is called to search for "flags" in the
buffer, but the return value is not checked before being used in pointer
arithmetic (flags - buffer). If strstr() returns NULL because "flags" is
not found in /proc/cpuinfo, this will cause undefined behavior and likely
a crash.

Add a NULL check after the strstr() call and handle the error appropriately
by cleaning up resources and reporting a meaningful error message.

Signed-off-by: Malaya Kumar Rout <mrout@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>