apps: avoid memory overrun.

NULL terminate the built in "help" argv array to avoid
reading beyond the end.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/12258)

apps/openssl.c

diff --git a/apps/openssl.c b/apps/openssl.c
index fdf4a74..3593c2b 100644
--- a/apps/openssl.c
+++ b/apps/openssl.c
@@ -205,7 +205,7 @@
 }
 #endif /* OPENSSL_NO_TRACE */
 
-static char *help_argv[] = { "help" };
+static char *help_argv[] = { "help", NULL };
 
 int main(int argc, char *argv[])
 {