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>

scripts/kconfig/mconf.c

@@ -357,8 +357,9 @@ static void get_symbol_str(struct gstr *r, struct symbol *sym)
 	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;

scripts/kconfig/util.c

@@ -84,12 +84,15 @@ void str_free(struct gstr *gs)
 /* Append to growable string */
 void str_append(struct gstr *gs, const char *s)
 {
-	size_t l = strlen(gs->s) + strlen(s) + 1;
-	if (l > gs->len) {
-		gs->s   = realloc(gs->s, l);
-		gs->len = l;
+	size_t l;
+	if (s) {
+		l = strlen(gs->s) + strlen(s) + 1;
+		if (l > gs->len) {
+			gs->s   = realloc(gs->s, l);
+			gs->len = l;
+		}
+		strcat(gs->s, s);
 	}
-	strcat(gs->s, s);
 }
 
 /* Append printf formatted string to growable string */