pci server crashes during boot on Qubes OS

I tried to launch Minix3 in Qubes OS. While there is no problem to boot
minix as a qube (in Qubes OS terminology) before 3641562, it fails with
the commit (and after). I didn't digg into PCI handling but this change
fixes the problem. Minix handles NULL case from pci_subclass_name.

Change-Id: I162424d92b613598e6eb845a71f90a02e31041db

sys/dev/pci/pci_subr.c

@@ -617,7 +617,11 @@ pci_subclass_name(pcireg_t reg)
 		subclassp++;
 	}
 
-	return subclassp->name;
+	if (subclassp) {
+		return subclassp->name;
+	} else {
+		return NULL;
+	}
 }
 #endif /* defined(__minix) && defined(_PCI_SERVER) */