Avoid segfault on CLI command "decodepay 1111111" (invalid short bech32 string)

Before this patch:

```
$ cli/lightning-cli decodepay 1111111111
"Invalid bolt11: Bad bech32 string"
$ cli/lightning-cli decodepay 111111111
"Invalid bolt11: Bad bech32 string"
$ cli/lightning-cli decodepay 11111111
"Invalid bolt11: Bad bech32 string"
$ cli/lightning-cli decodepay 1111111
lightning-cli: Non-object response ''
$ cli/lightning-cli decodepay 1111111
lightning-cli: Connecting to 'lightning-rpc': Connection refused
```

After this patch:

```
$ cli/lightning-cli decodepay 1111111111
"Invalid bolt11: Bad bech32 string"
$ cli/lightning-cli decodepay 111111111
"Invalid bolt11: Bad bech32 string"
$ cli/lightning-cli decodepay 11111111
"Invalid bolt11: Bad bech32 string"
$ cli/lightning-cli decodepay 1111111
"Invalid bolt11: Bad bech32 string"
$ cli/lightning-cli decodepay 1111111
"Invalid bolt11: Bad bech32 string"
```

common/bolt11.c

@@ -474,6 +474,9 @@ struct bolt11 *bolt11_decode(const tal_t *ctx, const char *str,
 
         b11->routes = tal_arr(b11, struct route_info *, 0);
 
+        if (strlen(str) < 8)
+                return decode_fail(b11, fail, "Bad bech32 string");
+
         hrp = tal_arr(tmpctx, char, strlen(str) - 6);
         data = tal_arr(tmpctx, u5, strlen(str) - 8);