Avoid assertion violation in case of kmm_free(NULL)

It is ok to call kmm_free with a NULL pointer.  Thus adopt the
DEBUGASSERT statement to cover this case.

Signed-off-by: Michael Jung <michael.jung@secore.ly>

mm/kmm_heap/kmm_free.c

@@ -52,7 +52,7 @@
 
 void kmm_free(FAR void *mem)
 {
-  DEBUGASSERT(kmm_heapmember(mem));
+  DEBUGASSERT((mem == NULL) || kmm_heapmember(mem));
   mm_free(g_kmmheap, mem);
 }