BE core when to_bitmap argument is invalid

[kangkaisen]

Today, BE core when our user test the bitmap_union:

terminate called after throwing an instance of 'std::out_of_range'
 what(): stoi
*** Aborted at 1567570614 (unix time) try "date -d @1567570614" if you are using GNU date ***
PC: @ 0x7f0b029f8207 __GI_raise
*** SIGABRT (@0x1f40001da70) received by PID 121456 (TID 0x7f0a4d280700) from PID 121456; stack trace: ***
 @ 0x7f0b036c25d0 (unknown)
 @ 0x7f0b029f8207 __GI_raise
 @ 0x7f0b029f98f8 __GI_abort
 @ 0x2cddde5 __gnu_cxx::__verbose_terminate_handler()
 @ 0x2c4fdc6 __cxxabiv1::__terminate()
 @ 0x2c4fe11 std::terminate()
 @ 0x2c863ee execute_native_thread_routine
 @ 0x7f0b036badd5 start_thread
 @ 0x7f0b02abfead __clone

So, we should add a check for the to_bitmap input argument.