ptos: mm: Add missing NULL checks

Signed-off-by: Chloe M. <chloe@mensia.org>

service/ptos/mm/pm.c

@@ -139,6 +139,10 @@ PmPfdListPop(MM_PFD_LIST *List)
     }
 
     FirstPfd = List->First;
+    if (FirstPfd == NULL) {
+        return NULL;
+    }
+
     List->First = FirstPfd->Next;
     return FirstPfd;
 }
@@ -219,8 +223,11 @@ MmRequestFrame(VOID)
     UPTR PhysicalBase;
 
     Pfd = PmPfdListPop(&AvlList);
-    RtlMemSet(Pfd, 0, PAGESIZE);
+    if (Pfd == NULL) {
+        return PFN_ERROR;
+    }
 
+    RtlMemSet(Pfd, 0, PAGESIZE);
     PhysicalBase = VMA_TO_PMA(Pfd);
     return ADDRESS_TO_PFN(PhysicalBase);
 }