sh(1): initialize smark to zero in main()

As popstackmark may be called on this without pushstackmark having
been called, we need to initialize it so that we don't get a bogus
comparison inside popstackmark, which would have resulted in a
NULL pointer dereference.

MFC After:	3 days
Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D40413

bin/sh/main.c

@@ -100,7 +100,7 @@ static char *find_dot_file(char *);
 int
 main(int argc, char *argv[])
 {
-	struct stackmark smark, smark2;
+	struct stackmark smark = {0}, smark2;
 	volatile int state;
 	char *shinit;