Factorize jump sanity checks from mark_jump_targets() and
draw_current_jump() in an is_valid_jump() function.

This fixes a segfault when moving the cursor over an invalid jump.
Signed-off-by: Frederik Deweerdt <frederik.deweerdt@xprog.eu>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20130114194716.GA4973@ks398093.ip-192-95-24.net
[ committer note: Make it a disasm_line method ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>