Fix warning introduced with last checkin

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3262 42af7a65-404d-4744-a932-0658087f49c3

mm/mm_memalign.c

@@ -100,7 +100,7 @@ FAR void *memalign(size_t alignment, size_t size)
   /* Then malloc that size */
 
   rawchunk = (size_t)malloc(allocsize);
-  if (rawchunk == NULL)
+  if (rawchunk == 0)
     {
       return NULL;
     }