x86/xen: panic on bad Xen-provided memory map

Panic if Xen provides a memory map with 0 entries. Although this is
unlikely, it is better to catch the error at the point of seeing the map
than later on as a symptom of some other crash.
Signed-off-by: NMartin Kelly <martkell@amazon.com>
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>

arch/x86/xen/setup.c

@@ -595,6 +595,7 @@ char * __init xen_memory_setup(void)
 		rc = 0;
 	}
 	BUG_ON(rc);
+	BUG_ON(memmap.nr_entries == 0);
 
 	/*
 	 * Xen won't allow a 1:1 mapping to be created to UNUSABLE