kconfig: fix segv fault in menuconfig

With specific configurations requesting help for certain
menu lines caused menuconfig to crash.
This was tracked down to a null pointer bug.
Thanks to "Miles Lane" <miles.lane@gmail.com> for inital reporting
and to Gabriel C <nix.or.die@googlemail.com> for the backtrace
that helped me locating the bug.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/scripts/kconfig/mconf.c b/scripts/kconfig/mconf.c
index 2ee12a7..1935818 100644
--- a/scripts/kconfig/mconf.c
+++ b/scripts/kconfig/mconf.c
@@ -357,8 +357,9 @@
 	bool hit;
 	struct property *prop;
 
-	str_printf(r, "Symbol: %s [=%s]\n", sym->name,
-	                               sym_get_string_value(sym));
+	if (sym && sym->name)
+		str_printf(r, "Symbol: %s [=%s]\n", sym->name,
+		                                    sym_get_string_value(sym));
 	for_all_prompts(sym, prop)
 		get_prompt_str(r, prop);
 	hit = false;