Feature/apdu null length returned (#285)

* Add APDU as NULL to get BACnet type lengths.

* Fix bacapp copy test to succeed

* fix BACnet REAL and DOUBLE decode

* Add unit test for NULL APDU encoding for length

* Add unit tests for bacapp context

* refactor host-n-port to hostnport.c module

* fix BVLC decoder

* additional unit testing for bacapp

* include bacdevobjpropref module in builds

* simplify bacapp snprintf to be able to return length

* adjust compiler for variable-length arrays

* fix bug found by scan-build

Authored-by: Steve Karg <skarg@users.sourceforge.net>

src/bacnet/bacint.h

@@ -100,9 +100,14 @@ extern "C" {
         uint8_t * buffer,
         uint64_t * value);
 #endif
+
     BACNET_STACK_EXPORT
     int bacnet_unsigned_length(
         BACNET_UNSIGNED_INTEGER value);
+    BACNET_STACK_EXPORT
+    int bacnet_signed_length(
+        int32_t value);
+
     /* signed value encoding and decoding */
     BACNET_STACK_EXPORT
     int encode_signed8(