Feature/comments and buffer checks (#73)

* Added comments

* Replaced Goto construct

* Added apdu_len check

* Added comments

* Added string limit and reworked printable check

* Mainly comments

* Just comments

* Just comments

* decode service request returns now non zero on success

* eliminated warnings

* Added character string init with length check.

* Paranoic length check

* Comments and object index checking on read/write.

* Check name/desc strings before returning.

* Eliminated Goto

src/bacnet/basic/npdu/h_npdu.c

@@ -70,6 +70,10 @@ void npdu_handler(BACNET_ADDRESS *src, /* source address */
     BACNET_ADDRESS dest = { 0 };
     BACNET_NPDU_DATA npdu_data = { 0 };
 
+    if (pdu_len < 1) {
+        return;
+    }
+
     /* only handle the version that we know how to handle */
     if (pdu[0] == BACNET_PROTOCOL_VERSION) {
         apdu_offset = npdu_decode(&pdu[0], &dest, src, &npdu_data);