Segmentation fault after recovering

[ZiyanShi]

Branch: pmem/kvdk/main:19332d5
Binary is built under kvdk/build/ by calling cmake .. and make -j
Then the old directory under pmem is cleared to create a new instance by calling
rm -rf /mnt/pmem0/debugging2
The new engine is created by
./bench -populate=1 -value_size=120 -threads=1 -time=10 -path=/mnt/pmem0/debugging2 -num=1048576 -space=209715200 -max_write_threads=1 -collections=1 -type=sorted -fill=1
and then closed.
Then the engine is reopened, trying to be restored, which will trigger segment error.
./bench -populate=1 -value_size=120 -threads=1 -time=10 -path=/mnt/pmem0/debugging2 -num=1048576 -space=209715200 -max_write_threads=1 -collections=1 -type=sorted -fill=0 -read_ratio=1

[peifeng si]

fix in #62