fix segmantaion fault in update_sdb()

fix segmantaion fault in update_sdb() while try to open corkami test file:

ltrace ./rabin2 -i ~/CoST.exe 
__libc_start_main(0x402501, 3, 0x7fffa86b6a38, 0x403ca0, 0x403c90 <unfinished ...>
r_str_home(0x404540, 0x7fffa86b6a38, 0x7fffa86b6a58, 0, 0x7f1dc8dd8320)                             = 0x878030
r_core_init(0x7fffa86554f0, 0, 0, 0, 0xfefefefefefefeff <unfinished ...>
--- SIGSEGV (Segmentation fault) ---
+++ killed by SIGSEGV +++