Fix a segfault in lash, hush, and cmdedit. Each of these used

xgetcwd, but did not check the return for a NULL, and then continued
to call strlen on the NULL when the cwd had been removed from under it.
 -Erik

shell/cmdedit.c

@@ -355,6 +355,10 @@ static void parse_prompt(const char *prmt_ptr)
 	char  c;
 	char *pbuf;
 
+	if (!pwd_buf) {
+		pwd_buf=unknown;
+	}
+
 	while (*prmt_ptr) {
 		pbuf    = buf;
 		pbuf[1] = 0;