remove an issue uncovered by clang analyzer

src/ua_types.c

@@ -178,6 +178,10 @@ UA_Guid UA_Guid_random(void) {
 
 /* ByteString */
 UA_StatusCode UA_ByteString_allocBuffer(UA_ByteString *bs, size_t length) {
+    if(length == 0) {
+        UA_ByteString_init(bs);
+        return UA_STATUSCODE_GOOD;
+    }
     if(!(bs->data = UA_malloc(length)))
         return UA_STATUSCODE_BADOUTOFMEMORY;
     bs->length = length;