before overwrite:
freelist[n-1] [0x003401b8] Flink 0x003401b8 Blink 0x003401b8
freelist[n]   [0x003401c0] Flink 0x00341fb0 Blink 0x00341fb0
chunk    [0x00341fa8] Flink 0x003401c0 Blink 0x003401c0

after overwrite:
freelist[n-1] [0x003401b8] same (Flink 0x003401b8 Blink 0x003401b8)
freelist[n]   [0x003401c0] same (Flink 0x00341fb0 Blink 0x00341fb0)
chunk    [0x00341fa8] Flink 0x003401bc Blink 0x003401c4

after 1st alloc (returned 0x00341fb0):
freelist[n-1] [0x003401b8] same (Flink 0x003401b8 Blink 0x003401b8)
freelist[n]   [0x003401c0] Flink 0x003401c4 Blink 0x003401bc

after 2nd alloc (returned 0x003401bc):
freelist[n-1] [0x003401b8] Flink 0x0008015c Blink 0x003401b8
freelist[n]   [0x003401c0] same (Flink 0x003401c4 Blink 0x003401bc)

After copying 0x909090909090909090 into newly allocated chunk
freelist[n-1] [0x003401b8] Flink 0x0008015c Blink 0x90909090
freelist[n]   [0x003401c0] Flink 0x90909090 Blink 0x90909090