Various minor fixes/improvements (#239)

Includes:
- Making "rpm_data_free" a public method and using it where appropriate
  - Includes fixing memory leak in epics demo application
    (note Valgrind indicates that there are more...)
- Fix typo in cov.c
- Allow NULL max_apdu parameter to address_get_by_device
- Add missing include to s_cov.h

src/bacnet/basic/binding/address.c

@@ -521,7 +521,9 @@ bool address_get_by_device(
             if ((pMatch->Flags & BAC_ADDR_BIND_REQ) == 0) {
                 /* If bound then fetch data */
                 bacnet_address_copy(src, &pMatch->address);
-                *max_apdu = pMatch->max_apdu;
+                if (max_apdu) {
+                    *max_apdu = pMatch->max_apdu;
+                }
                 /* Prove we found it */
                 found = true;
             }