Avoid NULL check in free() and malloc_usable_size().

Generalize isalloc() to handle NULL pointers in such a way that the NULL
checking overhead is only paid when introspecting huge allocations (or
NULL).  This allows free() and malloc_usable_size() to no longer check
for NULL.

Submitted by Igor Bukanov and Mike Hommey.

test/null.exp

@@ -0,0 +1,2 @@
+Test begin
+Test end