[fusl] Update fusl

fusl/src/malloc/malloc_usable_size.c

 #include <malloc.h>
 
 void *(*const __realloc_dep)(void *, size_t) = realloc;
 
 struct chunk {
         size_t psize, csize;
         struct chunk *next, *prev;
 };
 
 #define OVERHEAD (2*sizeof(size_t))
 #define CHUNK_SIZE(c) ((c)->csize & -2)
 #define MEM_TO_CHUNK(p) (struct chunk *)((char *)(p) - OVERHEAD)
 
 size_t malloc_usable_size(void *p)
 {
-        return CHUNK_SIZE(MEM_TO_CHUNK(p)) - OVERHEAD;
+        return p ? CHUNK_SIZE(MEM_TO_CHUNK(p)) - OVERHEAD : 0;
 }