hwpmc: Avoid panic on AMD cpus where IBS is not available

The recent IBS work intruduced a bug on older CPUs where the
IBS handler will be called on NMIs even when IBS is not initialized.
Work around this in the IBS handler by checking to see if ibs_pcpu
is NULL before accessing it.

sys/dev/hwpmc/hwpmc_ibs.c

@@ -510,6 +510,9 @@ pmc_ibs_intr(struct trapframe *tf)
 	int retval, cpu;
 	uint64_t config;
 
+	if (ibs_pcpu == NULL)
+		return (0);
+
 	cpu = curcpu;
 	KASSERT(cpu >= 0 && cpu < pmc_cpu_max(),
 	    ("[ibs,%d] out of range CPU %d", __LINE__, cpu));