[BOOT:MM] Fix descriptor memory leaks

Sometimes when handling MmMdAddDescriptorToList() errors, descriptors
were not freed with MmMdFreeDescriptor().

BOOT/ENVIRON/LIB/MM/mmpa.c

@@ -141,6 +141,7 @@ Return Value:
 
         Status = MmMdAddDescriptorToList(&MmMdlReservedAllocated, NewDescriptor, 0x00);
         if (!NT_SUCCESS(Status)) {
+            MmMdFreeDescriptor(NewDescriptor);
             return Status;
         }