Add buffer overflow protection to connect

Closes #492

src/PubSubClient.cpp

@@ -165,15 +165,21 @@ boolean PubSubClient::connect(const char *id, const char *user, const char *pass
 
             buffer[length++] = ((MQTT_KEEPALIVE) >> 8);
             buffer[length++] = ((MQTT_KEEPALIVE) & 0xFF);
+
+            CHECK_STRING_LENGTH(length,id)
             length = writeString(id,buffer,length);
             if (willTopic) {
+                CHECK_STRING_LENGTH(length,willTopic)
                 length = writeString(willTopic,buffer,length);
+                CHECK_STRING_LENGTH(length,willMessage)
                 length = writeString(willMessage,buffer,length);
             }
 
             if(user != NULL) {
+                CHECK_STRING_LENGTH(length,user)
                 length = writeString(user,buffer,length);
                 if(pass != NULL) {
+                    CHECK_STRING_LENGTH(length,pass)
                     length = writeString(pass,buffer,length);
                 }
             }

src/PubSubClient.h

@@ -83,6 +83,8 @@
 #define MQTT_CALLBACK_SIGNATURE void (*callback)(char*, uint8_t*, unsigned int)
 #endif
 
+#define CHECK_STRING_LENGTH(l,s) if (l+2+strlen(s) > MQTT_MAX_PACKET_SIZE) {_client->stop();return false;}
+
 class PubSubClient : public Print {
 private:
    Client* _client;