HS 2.0: Fix hs20_get_icon() error path

If base64_encode() fails (e.g., due to memory allocation failure), the
return value needs to be checked against NULL before using the (not
really) returned length value. The error path here ended up reading
uninitialized variable.

Signed-off-by: Jouni Malinen <j@w1.fi>

wpa_supplicant/hs20_supplicant.c

@@ -320,7 +320,7 @@ int hs20_get_icon(struct wpa_supplicant *wpa_s, const u8 *bssid,
 		return -1;
 
 	b64 = base64_encode(&icon->image[offset], size, &b64_size);
-	if (buf_len >= b64_size) {
+	if (b64 && buf_len >= b64_size) {
 		os_memcpy(reply, b64, b64_size);
 		reply_size = b64_size;
 	} else {