ltp: fix review questions

Signed-off-by: yangyalei <yangyalei@xiaomi.com>

mm/umm_heap/umm_malloc.c

@@ -62,7 +62,7 @@ FAR void *malloc(size_t size)
   /* Use mm_malloc() because it implements the clear */
 
   ret = mm_malloc(USR_HEAP, size);
-  if (!ret)
+  if (ret == NULL)
     {
       set_errno(ENOMEM);
     }

mm/umm_heap/umm_memalign.c

@@ -89,7 +89,7 @@ FAR void *memalign(size_t alignment, size_t size)
   FAR void *ret;
 
   ret = mm_memalign(USR_HEAP, alignment, size);
-  if (!ret)
+  if (ret == NULL)
     {
       set_errno(ENOMEM);
     }

mm/umm_heap/umm_realloc.c

@@ -91,7 +91,7 @@ FAR void *realloc(FAR void *oldmem, size_t size)
   FAR void *ret;
 
   ret = mm_realloc(USR_HEAP, oldmem, size);
-  if (!ret)
+  if (ret == NULL)
     {
       set_errno(ENOMEM);
     }

mm/umm_heap/umm_zalloc.c

@@ -68,7 +68,7 @@ FAR void *zalloc(size_t size)
   /* Use mm_zalloc() because it implements the clear */
 
   ret = mm_zalloc(USR_HEAP, size);
-  if (!ret)
+  if (ret == NULL)
     {
       set_errno(ENOMEM);
     }