Fix the backup API so that a backup from an empty database to a non-empty

database works.  Ticket [0bf974bdf9].  The only changes are in assert()
statements.

FossilOrigin-Name: ddb71cd9ed395804a13dc136bb7688a7627c798f

src/os_unix.c

@@ -3113,6 +3113,19 @@ static int unixTruncate(sqlite3_file *id, i64 nByte){
     ((unixFile*)id)->lastErrno = errno;
     return SQLITE_IOERR_TRUNCATE;
   }else{
+#ifndef NDEBUG
+    /* If we are doing a normal write to a database file (as opposed to
+    ** doing a hot-journal rollback or a write to some file other than a
+    ** normal database file) and we truncate the file to zero length,
+    ** that effectively updates the change counter.  This might happen
+    ** when restoring a database using the backup API from a zero-length
+    ** source.
+    */
+    if( ((unixFile*)id)->inNormalWrite && nByte==0 ){
+      ((unixFile*)id)->transCntrChng = 1;
+    }
+#endif
+
     return SQLITE_OK;
   }
 }