menuconfig: fix NULL pointer dereference when searching a symbol

Searching for PPC_EFIKA results in a segmentation fault, and it's
because get_symbol_prop() returns NULL.

In this case CONFIG_PPC_EFIKA is defined in arch/powerpc/platforms/
52xx/Kconfig, so it won't be parsed if ARCH!=PPC, but menuconfig knows
this symbol when it parses sound/soc/fsl/Kconfig:

    config SND_MPC52xx_SOC_EFIKA
        tristate "SoC AC97 Audio support for bbplan Efika and STAC9766"
        depends on PPC_EFIKA

This bug was introduced by commit bcdedcc1 ("menuconfig: print more
info for symbol without prompts").
Reported-and-tested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Tested-by: Libo Chen <libo.chen@huawei.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/kconfig/menu.c

@@ -600,7 +600,9 @@ void get_symbol_str(struct gstr *r, struct symbol *sym,
 	}
 	for_all_prompts(sym, prop)
 		get_prompt_str(r, prop, head);
+
 	prop = get_symbol_prop(sym);
+	if (prop) {
 		str_printf(r, _("  Defined at %s:%d\n"), prop->menu->file->name,
 			prop->menu->lineno);
 		if (!expr_is_yes(prop->visible.expr)) {
@@ -608,6 +610,8 @@ void get_symbol_str(struct gstr *r, struct symbol *sym,
 			expr_gstr_print(prop->visible.expr, r);
 			str_append(r, "\n");
 		}
+	}
+
 	hit = false;
 	for_all_properties(sym, prop, P_SELECT) {
 		if (!hit) {