Fix the 'is' command, which is broken due to a bad format string passed ...

The `is` command (used to list symbols) result to a segfault in the master branch and the stable branch, but not in the ArchLinux package (the version is 0.9.7).
I didn't find which commit broke it, but the segfault is due to a bad format string passed to r_cons_printf : a format PFMT64 is used to print ut32 integer.

This patch try to fix this behavior.

-TOSH-