[NTOS:MM] Fix broken ASSERT

There is no guarantee that all pages in the requested range actually exist.

ntoskrnl/mm/freelist.c

@@ -318,7 +318,7 @@ MiAllocatePagesForMdl(IN PHYSICAL_ADDRESS LowAddress,
                 // Get the PFN entry for this page
                 //
                 Pfn1 = MiGetPfnEntry(Page);
-                ASSERT(Pfn1);
+                if (!Pfn1) continue;
 
                 //
                 // Make sure it's free and if this is our first pass, zeroed