selinux: drop unnecessary NULL check

Commit e3489f89 ("selinux: kill selinux_sb_get_mnt_opts()")
introduced a NULL check on the context after a successful call to
security_sid_to_context().  This is on the one hand redundant after
checking for success and on the other hand insufficient on an actual
NULL pointer, since the context is passed to seq_escape() leading to a
call of strlen() on it.

Reported by Clang analyzer:

    In file included from security/selinux/hooks.c:28:
    In file included from ./include/linux/tracehook.h:50:
    In file included from ./include/linux/memcontrol.h:13:
    In file included from ./include/linux/cgroup.h:18:
    ./include/linux/seq_file.h:136:25: warning: Null pointer passed as 1st argument to string length function [unix.cstring.NullArg]
            seq_escape_mem(m, src, strlen(src), flags, esc);
                                   ^~~~~~~~~~~
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -1019,7 +1019,7 @@ static int show_sid(struct seq_file *m, u32 sid)
 	rc = security_sid_to_context(&selinux_state, sid,
 					     &context, &len);
 	if (!rc) {
-		bool has_comma = context && strchr(context, ',');
+		bool has_comma = strchr(context, ',');
 
 		seq_putc(m, '=');
 		if (has_comma)