x86/insn-eval: Make 0 a valid RIP for insn_get_effective_ip()

In theory, 0 is a valid value for the instruction pointer so don't use
it as the error return value from insn_get_effective_ip().

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210614135327.9921-5-joro@8bytes.org