RISC-V: Fix disassemble fetch fail return value.

RISC-V: Fix disassemble fetch fail return value.

This bug reported in
https://sourceware.org/bugzilla/show_bug.cgi?id=30184
And discussed in
https://sourceware.org/pipermail/binutils/2023-February/126213.html

We also checked the implementation of return value in arm and mips.
So this patch changes the return value to -1, that can fix bugs and maintain
consistency with other architectures.

opcodes/ChangeLog:

        * riscv-dis.c (print_insn_riscv):Change the return value.