linuxthreads/signals: do not restore handler for invalid signal
Invalid signals have no handlers so when trying to restore the old
handler to a bad signal a SIGSEGV occurs. This is because the library
tries to store the old handler to an invalid memory area where it things
the bad signal lives.
URL: https://bugs.busybox.net/show_bug.cgi?id=4640
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>